Solid oxide fuel cell system

ABSTRACT

The present invention provides a solid oxide fuel cell system capable of preventing excess temperature rises while increasing overall energy efficiency. The present invention is a solid oxide fuel cell system, including: a fuel cell module, a fuel supply device, a heat storing material, and a controller which, based on power demand, increases the fuel utilization rate when output power is high and to lower it when output power is low, and changes the electrical power actually output at a delay after changing the fuel supply amount. The controller has a stored heat estimating circuit for estimating the residual heat based on fuel supply and on power output at a delay relative thereto. When a utilizable amount of heat is accumulated in the heat storage material, the fuel supply is reduced so that the fuel utilization rate increases relative to the same electrical power.

RELATED APPLICATIONS

This application is a continuation of PCT/JP2011/072330 filed on Sep.29, 2011, which claims priority to the Japanese Application Nos.2010-218366 filed on Sep. 29, 2010, 2011-078888 filed on Mar. 31, 2011,2011-078889 filed on Mar. 31, 2011, and 2011-079464 filed on Mar. 31,2011. The entire contents of these applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a solid oxide fuel cell system, andmore particularly to a solid oxide fuel cell system for generatingvariable electrical power in response to power demand.

2. Description of the Related Art

Solid oxide fuel cells (“SOFCs” below) are fuel cells which operate atrelatively high temperatures in which, using an oxide ion-conductingsolid electrolyte as electrolyte, with electrodes attached to both sidesthereof, fuel gas is supplied to one side thereof and an oxidizer (air,oxygen, or the like) is supplied to the other side thereof.

In such SOFCs, steam or CO₂ is produced by the reaction between oxygenions passed through an oxide ion-conducting solid electrolyte and fuel,thereby generating electrical and thermal energy. The electrical energyis removed from the SOFC, where it is used for various electricalpurposes. On the other hand, thermal energy is used to raise thetemperature of the fuel, the reformer, the water, the oxidant, and thelike.

Unexamined Patent Application 2010-92836 (Patent Document 1) sets fortha fuel cell device. This fuel cell device is a solid oxide fuel cellsystem of the type which changes generated power in response to powerdemand; operation is disclosed whereby the fuel utilization rate isreduced more in the low load region than in the high power generationload region. That is, in Patent Document 1, the proportion of suppliedfuel used for power generation is reduced when generated power is in alow state, but on the other hand fuel used to heat the fuel cell moduleand not used to generate electricity is not greatly reduced, and a largefraction of the fuel is used to heat the fuel cell module, so the fuelcell module is made thermally independent, and a temperature at whichgeneration can occur is maintained.

Specifically, in the low generated power region, electrical generationheat occurring in the fuel cell unit associated with electricalgeneration declines, so there is a tendency for the temperature insidethe fuel cell module to decline, therefore if a certain fuel utilizationrate is maintained even in the low power generation region, a reductionin the temperature inside the fuel cell module is induced, and itbecomes difficult to maintain a temperature at which power can begenerated, therefore fuel used to heat the fuel cell module is increasedto enable thermal independence, even at the sacrifice of the fuelutilization rate.

In the fuel cell device set forth in Unexamined Patent Application2010-92836, in order to resolve these problems the fuel utilization rateis reduced in the low load region where electrical generation is small,preventing an excessive temperature reduction in the fuel cell modulewhile stably maintaining a fixed high temperature state.

Published Unexamined Patent Application 2009-104886 (Patent Document 2),on the other hand, sets forth a method of operation during increasingload on a fuel cell system. In this operating method, when increasingthe amount of electrical generation in a fuel cell system, the amount ofair supplied is first increased; then, after increasing the amounts ofwater and fuel in that order, the amount of electrical power extractedis increased. In this fuel cell system operating method, air depletion,carbon deposition, and fuel depletion are prevented from occurring byincreasing supply amounts in the order described.

In addition, the operating temperature of a solid oxide fuel cell isgenerally high, necessitating that fuel cell cells be kept at a highoperating temperature when generating. Therefore reducing the heatdispersed into the outside atmosphere from the fuel cells and reducingthe fuel required to maintain temperature is an important factor forincreasing the overall energy efficiency of a fuel cell system. It istherefore desirable that fuel cells be housed in a chassis with goodheat insulating properties.

Published Unexamined Patent Application 2010-205670 (Patent Document 3),on the other hand, sets an operating method for a fuel cell system andfuel cell. In this fuel cell system, an integral value for theelectrical load on the fuel cell is obtained, and the fuel utilizationrate is controlled based on this obtained integral value. Control of thefuel utilization rate is performed by estimating the fuel celltemperature based on the integral value of the fuel cell electricalload, then controlling the fuel utilization rate based on the estimatedresult. The fuel cell can therefore be operated in a thermallyindependent manner without use of a temperature sensor. When theintegral value of the electrical load is at or above a predeterminedvalue, the controller corrects the fuel utilization rate to a valueequal to or greater than a reference value at which the fuel cell canoperate with thermal independence. In this case, because of the factthat the flow chart temperature is rising, the flow chart has residualheat, and thermally self-sustaining operation can be maintained even ifthe fuel utilization rate is corrected to a value equal to or greaterthan the reference value at which thermal self-sustaining operation ispossible. The system efficiency of the fuel cell system is thusimproved.

3. Prior Art References

Patent Documents:

-   Patent Document 1—Published Unexamined Patent Application 2010-92836-   Patent Document 2—Published Unexamined Patent Application    2009-104886-   Patent Document 3—Published Unexamined Patent Application    2010-205670

SUMMARY OF THE INVENTION

There is a problem, however, in that notwithstanding efforts towardthermal self-sustaining operation, reducing fuel utilization rate in themanner described above results in an increase in fuel not contributingto power generation, such that operations reducing the fuel utilizationrate lead to a reduction in the overall energy efficiency of the solidoxide fuel cell system. Since overall energy efficiency is reduced inproportion to the length of operation in the reduced fuel utilizationrate state, this can also lead to a loss in the advantage of solid oxidefuel cells (SOFC), which are generally deemed to have a higher energyefficiency than polymer membrane (PEFC) fuel cells.

In particular, assuming the solid oxide fuel cell system is forresidential use, circumstances in which the solid oxide fuel cell systemis used in a low power generation state during predetermined hours ofthe day, such as during the night when occupants are asleep, etc., arecertain to occur; this greatly decreases the overall energy efficiencyof the solid oxide fuel cell system, leading to a call for solid oxidefuel cell system with superior technology capable of operating at a highfuel utilization rate even in such low power generation states.

On the other hand, by increasing thermal insulation on the chassis inwhich the fuel cell or the like is housed, residual fuel supplied to thefuel cells and not used for power generation heats up the chassis,causing the problem of excessive temperature rise inside the chassis.Damage to the fuel cells, reformer, and the like inside the chassis canoccur when the temperature inside the chassis rises excessively. It isnot easy to reduce excessively rising temperatures to an appropriatetemperature, since the chassis has high thermal insulationcharacteristics and extremely high thermal capacity.

If the fuel cell system is operated at a fixed generated power, theneven when thermal insulating characteristics of the chassis are high,excessive temperature rises can be avoided by setting the amount of fuelsupplied to a fixed amount so that thermal balance is maintained underthose high thermal insulating conditions. However, in a fuel cell inwhich variable generated power is produced according to power demand,the amount of fuel supplied must be changed to match generated power.When increasing generated power in fuel cell systems in which generatedpower is varied, the generated power extracted from the fuel cells mustbe increased by first increasing the amount of fuel supplied thenincreasing the power extracted from the fuel cell, as in the inventionset forth in Published Unexamined Patent Application 2009-104886.Therefore in fuel cell systems in which generated power is varied, whengenerated power is increased or decreased there is an increase inresidual fuel not contributing to power generation, which becomes acause for excessive temperature rises. Also, if the delay up until poweris extracted from the fuel cells is reduced in order to reduce residualfuel when increasing or decreasing power generation, the fuel cells areexposed to the risk of fuel depletion.

In response, Published Unexamined Patent Application 2010-205670describes utilizing residual heat accumulated in the fuel cell system toincrease the fuel utilization rate. When an excessive temperature riseoccurs in a fuel cell system, much of the residual heat is accumulated,so it is conceivable that a temperature rise can be suppressed byincreasing the fuel utilization rate and consuming the residual heat.However, in the invention set forth in Published Unexamined PatentApplication 2010-205670, the amount of stored residual heat is obtainedfrom the integral value of the electrical load, therefore the increasein residual fuel caused by increasing extracted generated power at adelay after increasing the fuel supply amount cannot be knownwhatsoever. It is therefore difficult to apply the invention ofPublished Unexamined Patent Application 2010-205670 to the suppressionof excessive temperature rises caused by power delays.

Specifically, in the invention set forth in Published Unexamined PatentApplication 2010-205670, the heat of generation (joule heat) arisingwhen power is generated in a fuel cell is estimated based on the amountof generated power; by this means residual heat is in turn estimated.Since heat generated by delaying power generation in a fuel cell iscombustion heat generated by the combustion of residual fuel not usedfor power generation, the amount of heat stored in the startup processwhen using the technology disclosed in Published Unexamined PatentApplication 2010-205670 cannot be estimated.

The present invention has the object of providing an extremely practicalsolid oxide fuel cell system capable of improving overall energyefficiency while maintaining thermal self-sufficiency and operating in astable manner.

The present invention has the further object of providing a solid oxidefuel cell system capable of preventing excessive temperature rises whileincreasing overall energy efficiency.

In order to resolve the above-described problems, the present inventionis a solid oxide fuel cell system for producing variable generated powerin accordance with power demand, comprising: a fuel cell module thatgenerates power using supplied fuel; a fuel supply device that suppliesfuel to the fuel cell module; a generating oxidant gas supply devicethat supplies oxidant gas for electrical generation to the fuel cellmodule; a heat storage material that stores heat produced within thefuel cell module; a demand power detection device that detects powerdemand; and a controller programmed to control the fuel supply devicebased on the demand power detected by the demand power detection deviceso that the fuel utilization rate increases when generated power islarge and decreases when generated power is small, wherein thecontroller is programmed to change the electrical power actually outputfrom the fuel cell module with a delay after changing the fuel supplyamount based on changes in demand power; wherein the controllercomprises a stored heat estimating circuit that estimates the amount ofsurplus heat based on fuel supplied by the fuel supply device and on thepower output at a delay relative to fuel supply, and wherein when thestored heat estimating circuit estimates that a utilizable amount ofheat has accumulated in the storage material, the controller reduces thefuel supply amount so that the fuel utilization rate for the samegenerated power is increased relative to the case when a utilizableamount of heat has not accumulated.

In the present invention thus constituted, the fuel supply device andgenerating oxidant gas supply device respectively supply fuel andgenerating oxidant gas to the fuel cell module. The fuel cell modulegenerates electricity using the supplied fuel and generating oxidantgas, and the heat produced is stored in the heat storage material. Basedon the demand power detected using the demand power detection device,the controller controls the fuel supply device so that the fuelutilization rate is high when the generated power is large, and the fuelutilization rate is low when the generated power is small. In addition,the controller changes the power actually output from the fuel cellmodule at a delay after changing the fuel supply amount in response tochanges in power demand. The stored heat amount estimating circuitestimates a surplus heat amount based on the fuel supply and on thepower output at a delay relative to the supply of fuel. When it isestimated by the stored heat estimating circuit that a utilizable amountof heat is stored in the storage material, the controller reduces thefuel supply amount so that the fuel utilization rate is higher for agenerated power than when a utilizable amount of heat is not stored.

In general, in solid oxide fuel cell system electrical generation heatdeclines when generated power is small, facilitating a decline in thetemperature of the fuel cell module. Therefore at times of low powergeneration, the fuel utilization rate is reduced and fuel not used ingenerating electricity is combusted to heat up the fuel cell module andprevent excessive temperature drops. In particular, in solid oxide fuelcell system of the type in which a reformer is disposed within the fuelcell module, an endothermic reaction occurs inside the reformer,facilitating an even further reduction in temperature. In the presentinvention thus constituted, when it is estimated by the stored heatestimating circuit that a utilizable amount of heat is stored in thestorage material, the fuel supply amount is reduced in order to increasethe fuel utilization rate. This enables the overall energy efficiency ofthe solid oxide fuel cell system to be improved while maintainingthermal self-sufficiency of the solid oxide fuel cell system andavoiding excessive drops in temperature.

Also, in the present invention thus constituted, surplus heat amountsare estimated based on the fuel supplied by the fuel supply device andon power output at a delay relative to the supply of fuel, so theaccumulated heat amount can also be accurately estimated by having thecontroller change the fuel supply amount then, at a delay, change theoutput power. Therefore the amount of heat stored in the heat storagematerial can be sufficiently exploited while reliably avoiding the riskof sudden temperature drops in the fuel cell module. In addition, infuel cells of the type in which output power is varied at a delay afterchanging the fuel supply amount, there is a risk that frequent increasesand decreases of output power will cause an excessive rise intemperature within the fuel cell module, but according the presentinvention constituted as described above, stored heat caused by surplusfuel produced as described above can be accurately known. In general,excessive temperature rise caused by surplus heat is suppressed byinserting a cooling rod body into the fuel cell module, but using thepresent invention the amount of heat resulting from surplus fuel can beaccurately known, and can therefore be effectively used to suppressexcessive temperature rises. By this means the amount of cooling mediuminserted to reduce the temperature can be reduced, thereby improving theoverall energy efficiency of the solid oxide fuel cell system.

In the present invention, the controller preferably greatly raises thefuel utilization rate as the stored heat amount estimated by the storedheat estimating circuit increases.

In the present invention thus constituted, stored heat is used in largequantity when the estimated stored heat is large, and is not much usedwhen the stored heat amount is small, therefore stored heat can beeffectively exploited, and the risk of temperature drops can be reliablyavoided.

In the present invention, the controller preferably makes much greaterchanges in the fuel utilization rate relative to changes in theestimated stored heat amount in the region where the amount of storedheat estimated by the stored heat estimating circuit is large than inthe region where the estimated stored heat amount is small.

In the present invention thus constituted, when the estimated storedheat amount is large, large amounts of stored heat are used to avoidexcessive temperature rises, while when the estimated stored heat amountis small, small amount of stored heat are used at a time to preventovercooling.

In the present invention, the stored heat estimating circuit preferablyestimates a stored heat amount by summing addition and subtractionvalues reflecting the surplus heat amount caused by the output of powerat a delay relative to fuel supply.

In the present invention thus constituted, the stored heat amount isestimated by summing addition and subtraction values reflecting asurplus heat amount, therefore the stored heat amount resulting from thestorage of produced surplus heat can be precisely estimated.

In the present invention, the addition and subtraction values arepreferably determined based on the temperature inside the fuel cellmodule, the surplus heat amount calculated using the relationshipbetween fuel supply amount and generated power, the amount ofincrease/decrease in generated power, or the number of times generatedpower is increased/decreased per hour.

In the present invention thus constituted, addition and subtractionvalues are determined based on the temperature inside the fuel cellmodule, the surplus heat amount calculated using the relationshipbetween the fuel supply amount and generated power, the amount ofincrease/decrease in generated power, or the number of times generatedpower is increased/decreased per hour, therefore the stored heatproduced by delaying the produced electrical power can be accuratelyestimated.

In the present invention, the controller preferably controls the fuelsupply device so that when a utilizable amount of heat has notaccumulated in the heat storage material, a greater amount of heat isstored in the heat storage material in a region greater than apredetermined medium generated power, so that heat amounts accumulatedduring large power generation can be utilized during small powergeneration.

In the present invention thus constituted, a larger heat amount isstored in the heat storage material in a region above medium generatedpower, therefore by actively storing heat in a region above the mediumelectrical generation level where the fuel utilization rate can beincreased, this heat can be consumed during low power generation whenthe fuel cell module temperature is relatively low and self-sustainingis difficult, thus reliably enabling high efficiency operation at a highfuel utilization rate with effective use of the stored heat amount.

In the present invention, the controller preferably controls the fuelsupply device so that a larger amount of heat is stored in the heatstorage material in the region where generated power is greater than themiddle value of the generated power range.

In the present invention thus constituted, a larger amount of heat isstored in the heat storage material in the region where generated poweris greater than in the middle value of the generated power range.Therefore in the vicinity of the middle value of the frequently usedgenerated power range, the amount of stored surplus heat is suppressed,and a large amount of heat is stored in the heat storage material duringpower demand peaks. Thus when a solid oxide fuel cell system is used ina residence, excessive fuel consumption to store large heat amounts issuppressed during the periods of most frequent power demand amounts,being the medium level power demand amounts occurring during the day,etc., while on the other hand large heat amounts are stored during timeperiods with peak power demand, such as evening hours, so that heatamounts stored in the evening hours are immediately consumed in thefollow-on late night period; wasteful storage of heat amounts over longperiods is eliminated, and a high efficiency operation can be achievedto reliably take effective advantage of stored heat during the latenight period when generated power is greatly reduced.

In the present invention, the controller preferably increases the fuelutilization rate when the stored heat amount estimated by the storedheat estimating circuit is equal to or greater than a predeterminedchange execution stored heat amount.

In the present invention thus constituted, the stored heat amount in theheat storage material is estimated by the stored heat estimatingcircuit, therefore changes to increase the fuel utilization rate can bestably executed, and a change is executed when the estimated stored heatamount is equal to or greater than a predetermined change-executionstored heat amount, so that overcooling can be more reliably prevented.

In the present invention, the controller preferably determines apredetermined change execution period based on the stored heat amountestimated by the stored heat estimating circuit at the start of highefficiency control at an increased fuel utilization rate, and executeshigh efficiency control within this change execution period.

In the present invention thus constituted, changes are executed withinthe change execution period determined based on the stored heat amountestimated by the stored heat estimating circuit, so that high efficiencycontrol utilizing stored heat can be effected using a simpler control.

The present invention preferably further comprises a change periodextension circuit for suppressing decreases in the amount of heat storedin the heat storage material to extend the period of execution of highefficiency control during execution of high efficiency control at anincreased fuel utilization rate.

The present invention thus constituted is furnished with a change periodextension circuit for extending the period for executing high efficiencycontrol, therefore the stored heat amount can be effectively utilized inaccordance with conditions.

In the present invention, the change period extension circuit preferablydecreases the fuel utilization rate in proportion to the lengthening ofthe period during which the high efficiency control is executed, inconjunction with the decrease in the amount of stored heat stored in theheat storage material.

In the present invention thus constituted, the amount of change in highefficiency control is reduced with the decrease in stored heat amount,therefore the period during which the fuel utilization rate is increasedcan be extended without inducing excessive temperature drops in the fuelcell module, degradation of performance, or the like.

In the present invention, the change period extension circuit preferablydecreases the fuel utilization rate in proportion to the decrease ingenerated power.

In the present invention thus constituted, because the change amountunder high efficiency control is reduced more as generated powerdecreases, there is a decrease in the change amount during low powergeneration, in which the amount of stored heat utilized increases, andthe period during which the fuel utilization rate is increased can beextended while reliably avoiding excessive temperature drops in the fuelcell module, degradation of performance, or the like.

In the present invention, the change period extension circuit preferablycontrols the generating oxidant gas supply device to reduce oxidant gasfor generation supplied to the fuel cell module while high efficiencycontrol is being executed.

In the present invention thus constituted, generating oxidant gassupplied to the fuel cell module is reduced during change execution,therefore the carrying off of the heat amount stored in the heat storagematerial by oxidant gas can be suppressed, and stored heat can beeffectively used over a longer time period.

The present invention preferably further comprises an overcoolingprevention circuit that prevents overcooling of the fuel cell modulewhen the stored heat amount in the heat storage material is small.

The present invention thus constituted is furnished with an overcoolingprevention circuit, therefore overcooling caused by increasing the fuelutilization rate can be reliably prevented in a state in which theamount of stored heat has declined.

In the present invention, during execution of high efficiency controlwith an increased fuel utilization rate, the overcooling preventioncircuit preferably improves the fuel supply amount followingcharacteristics of the fuel supply device more than during normaloperation.

In the present invention thus constituted, the fuel supply amountfollowing characteristics are improved during the period when highefficiency control is being executed, therefore the fuel supply amountcan be quickly increased when the fuel utilization rate drops with adecline in the stored heat amount. Overcooling of the fuel cell moduledue to delays in response which cause an increase in the fuel supplyamount can thus be prevented.

The present invention preferably further comprises a combustion portionfor heating the fuel cell module by combusting residual fuel, which isremaining fuel supplied by the fuel supply device and not used for powergeneration; wherein the controller further includes: a power extractiondelay circuit which, when generated power is increased, increases thefuel supply amount supplied to the fuel cell module, then increases thepower extracted from the fuel cell module after a delay; an excesstemperature rise estimating circuit that estimates the occurrence ofexcessive temperature rises inside the fuel cell module; a temperaturerise suppression circuit which, when the occurrence of an excessivetemperature rise is estimated by the excess temperature rise estimatingcircuit, suppresses temperature rises in the fuel cell module whilecontinuing power generation by reducing the residual fuel produced bythe delay of output power provided by the power extraction delaycircuit; and a forced cooling circuit for lowering the temperatureinside the fuel cell module by causing a cooling fluid to flow into thefuel cell module when further temperature rise suppression is requiredafter executing temperature rise suppression using the temperature risesuppression circuit.

In the present invention thus constituted, the fuel supply device andgenerating oxidant gas supply device respectively supply fuel andgenerating oxidant gas to the fuel cell module. The fuel cell modulegenerates electricity using the supplied fuel and generating oxidantgas, and residual fuel unused for generation and remaining is combustedin a combustion portion such that the inside of the fuel cell module isheated. The controller controls the fuel supply device based on a demandpower detected by a demand power detection device. Also, the powerextraction delay circuit with which the controller is furnished changesthe power actually output from the fuel cell module at a delay afterchanging the fuel supply amount in response to changes in power demand.The temperature rise suppression circuit with which the controller isprovided suppresses temperature rises in the fuel cell module whilecontinuing power generation by reducing the residual fuel produced bythe output at a delay of electrical power by the power extraction delaycircuit when the occurrence of an excessive temperature rise isestimated by the excess temperature rise estimating circuit. Moreover,the forced cooling circuit with which the controller is provided reducesthe temperature inside the fuel cell module by causing a cooling fluidto flow into the fuel cell module when further temperature risesuppression is required after executing temperature rise suppressionusing the temperature rise suppression circuit.

In the present invention thus constituted, output power can be changedafter changing the fuel supply using the power extraction delay circuitto secure a safe period of time for fuel to be dispersed, therefore therisk of damage to cells inside the fuel cell module due to fueldepletion can be avoided. In addition, residual fuel is increased bydelaying the output of power, and this residual fuel heats the interiorof the fuel cell module. When the heat insulating characteristics of thefuel cell module are high and there is a requirement to implementexcessive load following characteristics in which output power isfrequently increased and decreased, it can occur that excessivetemperature rises are induced in the fuel cell module. In general, theamount of generating oxidant gas as cooling medium is increased in orderto reduce the temperature inside the fuel cell module, but because thedrop in temperature caused by injecting a cooling medium is achieved bydischarging useful amounts of heat in the fuel cell module together withexhaust, the overall energy efficiency declines. In addition, thetemperature rise suppression circuit is constituted to reduce residualfuel produced by the delay in the output of power by the powerextraction delay circuit. Thus by suppressing the emission of heatcaused by combustion of residual fuel, excessive temperature rises canbe quickly reduced while continuing electrical generation. Temperaturerises can thus be suppressed while avoiding drops in energy efficiency.Furthermore, since the forced cooling circuit causes a cooling fluid toflow into the fuel cell module as needed to reduce the temperature aftersuppression of a temperature rise is executed by the temperature risesuppression circuit, excessive temperature rises can be reliablyavoided.

In the present invention, the temperature rise suppression circuitpreferably controls temperature rise inside the fuel cell module byincreasing the fuel utilization rate; and wherein the controllerdetermines whether or not to execute a temperature rise suppression bythe forced cooling circuit based on changes in the temperature insidethe fuel cell module after a temperature rise suppression has beenexecuted by the temperature rise suppression circuit.

In the present invention thus constituted, the temperature risesuppression circuit increases the fuel utilization rate, therefore theamount of heat stored inside the fuel cell module can be consumedwithout loss of energy efficiency; cooling by the forced cooling circuitis then executed based on temperature changes in the fuel cell moduleafter execution of the temperature rise suppression circuit, thereforeuse of the forced cooling circuit causing a decrease in energyefficiency can be kept to the minimum required.

In the present invention, the temperature rise suppression circuitincreases the fuel utilization rate and suppresses temperature rises inthe fuel cell module by reducing the frequency with which generatedpower is increased and decreased when following fluctuations in demandpower.

In the present invention thus constituted, the fuel utilization rate isimproved and the frequency of increases/decreases in generated power isreduced, so the stored heat amount is consumed and the production ofresidual fuel is suppressed, such that excess temperature rises can bequickly eliminated.

In the present invention, the forced cooling circuit increases the flowamount of oxidant gas supplied by the generating oxidant gas supplydevice and utilizes the additional oxidant gas as a fluid body forcooling.

In the present invention thus constituted, generating oxidant gas isincreased when excess temperature rises cannot be sufficientlysuppressed by the temperature rise suppression circuit, thereforetemperature rises can be quickly suppressed without negatively affectingcells inside the fuel cell module or the like.

The present invention preferably further comprises a combustion portionfor heating the fuel cell module by combusting residual fuel, which isremaining fuel supplied by the fuel supply device and not used for powergeneration; and a temperature detection device for detecting thetemperature of the fuel cell module; wherein the stored heat estimatingcircuit estimates the stored heat amount stored in the heat storagematerial based on the detected temperature detected by the temperaturedetection device; wherein the controller includes a power extractiondelay circuit that increases the generated power output from the fuelcell module at a delay after increasing the fuel supply amount suppliedto the fuel cell module when increasing generated power; wherein thecontroller includes a fuel supply amount change circuit that executeshigh efficiency control to reduce the fuel supply amount so that thefuel utilization rate rises, thereby causing the heat amount stored inthe heat storage material to be consumed; and wherein the controllerincludes a temperature rise suppression circuit that suppressestemperature rises by reducing the upper limit value in a variable rangeof power generated by the fuel cell module.

In the present invention thus constituted, the fuel supply device andgenerating oxidant gas supply device respectively supply fuel andgenerating oxidant gas to the fuel cell module. The fuel cell modulegenerates electricity using the supplied fuel and generating oxidantgas, and residual fuel unused for generation and remaining is combustedin a combustion portion such that the inside of the fuel cell module isheated. The controller controls the fuel supply device based on a demandpower detected by a demand power detection device. Also, the powerextraction delay circuit with which the controller is furnished changesthe power actually output from the fuel cell module at a delay afterchanging the fuel supply amount in response to changes in power demand.Furthermore, the stored heat estimating circuit estimates the amount ofstored heat stored in the heat storage material based on the temperaturedetected by the temperature detection device. When the estimated storedheat amount is large and occurrence of an excessive temperature rise inthe fuel cell module is foreseen, the fuel supply amount change circuitincreases the fuel utilization rate and performs high efficiency controlto cause the heat amount stored in the heat storage material to beconsumed. In addition, the temperature rise suppression circuitsuppresses temperature rises by lowering the upper limit value in thevariable range of power generated by the fuel cell module.

In the present invention thus constituted, output power can be changedafter changing the fuel supply using the power extraction delay circuitto secure a safe period of time for fuel to be dispersed, therefore therisk of damage to cells inside the fuel cell module due to fueldepletion can be avoided. In addition, residual fuel is increased bydelaying the output of power, and this residual fuel heats the interiorof the fuel cell module. When the heat insulating characteristics of thefuel cell module are high and there is a requirement to implementexcessive load following characteristics in which output power isfrequently increased and decreased, it can occur that excessivetemperature rises are induced in the fuel cell module. In general, theamount of generating oxidant gas as cooling medium is increased in orderto reduce the temperature inside the fuel cell module, but because thedrop in temperature caused by injecting a cooling medium is achieved bydischarging useful amounts of heat in the fuel cell module together withexhaust, overall energy efficiency declines. The solid oxide fuel cellsystem of the present invention suppresses excessive temperature risesby reducing the introduced fuel supply amount so that excessivelyaccumulated surplus heat is utilized as a thermal self-sustaining stateis maintained, while simultaneously achieving a high fuel utilizationrate. In order to achieve this, in the present invention the stored heatestimating circuit estimates the stored heat amount based on detectedtemperature, therefore the effects of stored heat caused by residualfuel resulting from a delay in the output of electrical power from thestart of fuel supply can be accurately accounted for and estimated.Excessive temperature rises occurring at times of excessive loadfollowing can thus be reliably prevented while energy efficiency isincreased. Furthermore, the temperature rise suppression circuit isconstituted to reduce the upper limit value of the variable range ofgenerated power. The amount of heat emission associated with electricalgeneration is thus suppressed, therefore further increases in the storedheat amount are suppressed, and since the range of variability inelectrical power caused by load following is also suppressed, theoccurrence of further heat amounts caused by residual fuel issuppressed, and excessive temperature rises can be quickly reduced.Moreover, since fuel utilization rates are high to begin with in highelectrical generation states, the amount of consumption of stored heatcaused by raising the fuel utilization rate is small, and suppression ofexcessive temperature rises takes a long period of time. There is also arisk of inducing further excessive temperature rises during this timeperiod. The present invention, by forcibly reducing the upper limit ofgenerated power, increases the stored heat carried off by increasing thefuel utilization rate so that surplus heat amounts can be activelyconsumed in a short time period, therefore excess temperature rises canbe reliably and promptly resolved.

EFFECT OF THE INVENTION

Using the solid oxide fuel cell system of the present invention, overallenergy efficiency can be improved while maintaining thermalself-sufficiency and stable operation.

Using the solid oxide fuel cell system of the present invention,excessive temperature rises can be prevented while increasing overallenergy efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overview diagram showing a fuel cell device according to anembodiment of the present invention.

FIG. 2 is a front elevation cross section showing a fuel cell module ina fuel cell device according to an embodiment of the present invention.

FIG. 3 is a sectional diagram along line III-III in FIG. 2.

FIG. 4 is a partial cross section showing a fuel cell unit in a fuelcell device according to an embodiment of the present invention.

FIG. 5 is an oblique view showing a fuel cell stack in a fuel celldevice according to an embodiment of the present invention.

FIG. 6 is a block diagram showing a fuel cell device according to anembodiment of the present invention.

FIG. 7 is a timing chart showing the operation when a fuel cell deviceis started, according to an embodiment of the present invention.

FIG. 8 is a timing chart showing the operation when a fuel cell deviceis stopped, according to an embodiment of the present invention.

FIG. 9 is a graph showing the relationship between output current andfuel supply amount in the solid oxide fuel cell system of the firstembodiment of the present invention.

FIG. 10 is a graph showing the relationship between output current andamount of heat produced by supplied fuel in the solid oxide fuel cellsystem of the first embodiment of the present invention.

FIG. 11 is a control flow chart of the fuel supply amount in the solidoxide fuel cell system of the first embodiment of the present invention.

FIG. 12 is a stored heat amount estimate table used to estimate theamount of heat accumulated in a heat storing material in the solid oxidefuel cell system of the first embodiment of the present invention.

FIG. 13 is a graph of the stored heat amount estimate table in FIG. 12.

FIG. 14 is a graph showing the value of a first modifying coefficientrelative to output current in the solid oxide fuel cell system of thefirst embodiment of the present invention.

FIG. 15 is a graph showing the value of a second modifying coefficientrelative to output current in the solid oxide fuel cell system of thefirst embodiment of the present invention.

FIG. 16 is a flowchart for changing correction amounts when the fuelcell module has degraded.

FIGS. 17( a) and 17(b) are graphs schematically showing changes in powerdemand over a day in a typical residence.

FIG. 18 is a graph showing the value of a current modifying coefficientin a variant example of the first embodiment of the present invention.

FIG. 19 is a graph schematically showing the relationship betweenchanges in power demand, fuel supply amount, and current actuallyextracted from a fuel cell module.

FIG. 20 is a graph showing an example of the relationship betweengenerating air supply amount, water supply amount, fuel supply amount,and current actually extracted from a fuel cell module.

FIG. 21 is a flowchart showing the order in which generating air supplyamount, water supply amount, and fuel supply amount are determined basedon detected temperature Td.

FIG. 22 is a graph showing appropriate fuel cell stack temperatureversus generating current.

FIG. 23 is a graph showing fuel utilization rate determined according tointegral value.

FIG. 24 is a graph showing the range of fuel utilization rates which canbe determined relative to each generating current.

FIG. 25 is a graph showing air utilization rates determined according tointegral value.

FIG. 26 is a graph showing the range of air utilization rates which canbe determined relative to each generating current.

FIG. 27 is a graph for determining water supply amounts versus adetermined air supply utilization rate.

FIG. 28 is a graph showing appropriate fuel cell module generatingvoltage versus generating current.

FIG. 29 is a flowchart showing the procedure for limiting the range ofpower produced by the fuel cell module in a second embodiment of thepresent invention.

FIG. 30 is a map showing current limits versus generating current anddetected temperature.

FIG. 31 is a timing chart showing an example of the effect of the secondembodiment of the present invention.

FIG. 32 is a graph showing an example of the relationship betweentemperature inside the fuel cell module and maximum generatable power.

FIG. 33 is a flow chart showing a procedure for calculating a firstadd/subtract value based on temperatures detected by multipletemperature sensors.

FIG. 34 is a flow chart showing the procedure for calculating anadd/subtract value according to a variant example of the secondembodiment of the present invention.

FIG. 35 is a flow chart showing the procedure for calculating anadd/subtract value according to a variant example of the secondembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, referring to the attached drawings, we discuss a solid oxide fuelcell system (SOFC) according to an embodiment of the present invention.

FIG. 1 is an overview diagram showing a solid oxide fuel cell system(SOFC) according to an embodiment of the present invention. As shown inFIG. 1, the solid oxide fuel cell system (SOFC) of this embodiment ofthe present invention is furnished with a fuel cell module 2 and anauxiliary unit 4.

The fuel cell module 2 is furnished with a housing 6; a sealed space 8is formed within the housing 6, mediated by an insulating material 7. Afuel cell assembly 12 for carrying out the electrical generatingreaction between fuel gas and oxidizer (air) is disposed in thegenerating chamber 10 at the lower portion of this sealed space 8. Thisfuel cell assembly 12 is furnished with ten fuel cell stacks 14 (seeFIG. 5), and a fuel cell stack 14 comprises 16 fuel cell units 16 (seeFIG. 4). Thus, the fuel cell assembly 12 has 160 fuel cell units 16, allof which are serially connected.

A combustion chamber 18 is formed above the aforementioned generatingchamber 10 in the fuel cell module 2 sealed space 8; residual fuel gasand residual oxidizer (air) not used in the electrical generationreaction are burned in this combustion chamber 18 and produce exhaustgas.

A reformer 20 for reforming fuel gas is disposed at the top of thecombustion chamber 18; the reformer 20 is heated by the heat of residualgas combustion to a temperature at which the reforming reaction can takeplace. Furthermore, an air heat exchanger 22 is disposed on the top ofthis reformer 20 for receiving heat from the reformer 20 and heating airso as to restrain temperature drops in the reformer 20.

Next, the auxiliary unit 4 is furnished with a pure water tank 26 forholding water from a municipal or other water supply source 24 andfiltering it into pure water, and a water flow regulator unit 28 (a“water pump” or the like driven by a motor) for regulating the flowvolume of water supplied from the reservoir tank. The auxiliary tank 4is further furnished with a gas shutoff valve 32 for shutting off thefuel gas supply from a fuel supply source 30 such as municipal gas orthe like, and a fuel flow regulator unit 38 (a “fuel pump” or the likedriven by a motor) for regulating the flow volume of fuel gas.Furthermore, an auxiliary unit 4 is furnished with an electromagneticvalve 42 for shutting off air serving as an oxidizer and supplied froman air supply source 40, a reforming air flow regulator unit 44 andoxidant gas supply device 45 (“air blower” or the like driven by amotor) for regulating air flow volume, a first heater 46 for heatingreforming air supplied to the reformer 20, and a second heater 48 forheating generating air supplied to the generating chamber. This firstheater 46 and second heater 48 are provided in order to efficientlyraise the temperature at startup, and may be omitted.

Next, a hot-water producing device 50 supplied with exhaust gas isconnected to the fuel cell module 2. Municipal water from a water supplysource 24 is supplied to this hot-water producing device 50; this wateris turned into hot water by the heat of the exhaust gas, and is suppliedto a hot water reservoir tank in an external water heater, not shown.

A control box 52 for controlling the amount of fuel gas supplied, etc.,is connected to the fuel cell module 2.

Furthermore, an inverter 54 serving as an electrical power extractionunit (electrical power conversion unit) for supplying electrical powergenerated by the fuel cell module to the outside is connected to thefuel cell module 2.

Next, the internal structure of the solid oxide fuel cell system (SOFC)fuel cell module of this embodiment of the present invention isexplained using FIGS. 2 and 3. FIG. 2 is a side elevation sectionaldiagram showing a fuel cell module in a solid oxide fuel cell system(SOFC) according to an embodiment of the present invention; FIG. 3 is asectional diagram along line III-III of FIG. 2.

As shown in FIGS. 2 and 3, starting from the bottom in the sealed space8 within the fuel cell module 2 housing 6, a fuel cell assembly 12, areformer 20, and an air heat exchanger 22 are arranged in sequence, asdescribed above.

A pure water guide pipe 60 for introducing pure water on the upstreamend of the reformer 20, and a reform gas guide pipe 62 for introducingthe fuel gas and reforming air to be reformed, are attached to thereformer 20; a vaporizing section 20 a and a reforming section 20 b areformed in sequence starting from the upstream side within the reformer20, and the steam generating section 20 a reforming section 20 b isfilled with a reforming catalyst. Fuel gas and air blended with thesteam (pure water) introduced into the reformer 20 is reformed by thereforming catalyst used to fill in the reformer 20. Appropriatereforming catalysts are used, such as those in which nickel is impartedto the surface of aluminum spheres, or ruthenium is imparted to aluminumspheres.

A fuel gas supply line 64 is connected to the downstream end of thereformer 20; this fuel gas supply line 64 extends downward, then furtherextends horizontally within a manifold formed under the fuel cellassembly 12. Multiple fuel supply holes 64 b are formed on the bottomsurface of a horizontal portion 64 a of the fuel gas supply line 64;reformed fuel gas is supplied into the manifold 66 from these fuelsupply holes 64 b.

A lower support plate 68 provided with through holes for supporting theabove-described fuel cell stack 14 is attached at the top of themanifold 66, and fuel gas in the manifold 66 is supplied into the fuelcell unit 16.

Next, an air heat exchanger 22 is provided over the reformer 20. Thisair heat exchanger 22 is furnished with an air concentration chamber 70on the upstream side and two air distribution chambers 72 on thedownstream side; this air concentration chamber 70 and the distributionchambers 72 are connected using six air flow conduits 74. Here, as shownin FIG. 3, three air flow conduits 74 form a set (74 a, 74 b, 74 c, 74d, 74 e, 74 f); air in the air concentration chamber 70 flows from eachset of the air flow conduits 74 to the respective air distributionchambers 72.

Air flowing in the six air flow conduits 74 of the air heat exchanger 22is pre-heated by rising combustion exhaust gas from the combustionchamber 18.

Air guide pipes 76 are connected to each of the respective airdistribution chambers 72; these air guide pipes 76 extend downward,communicating at the bottom end side with the lower space in thegenerating chamber 10, and introducing preheated air into the generatingchamber 10.

Next, an exhaust gas chamber 78 is formed below the manifold 66. Asshown in FIG. 3, an exhaust gas conduit 80 extending in the verticaldirection is formed on the insides of the front surface 6 a and the rearsurface 6 b which form the faces in the longitudinal direction of thehousing 6; the top inside of the exhaust gas conduit 80 communicateswith the space in which the air heat exchanger 22 is disposed, and thebottom end side communicates with the exhaust gas chamber 78. An exhaustgas discharge pipe 82 is connected at approximately the center of thebottom surface of the exhaust gas chamber 78; the downstream end of thisexhaust gas discharge pipe 82 is connected to the above-described hotwater producing device 50 shown in FIG. 1.

As shown in FIG. 2, an ignition device 83 for starting the combustion offuel gas and air is disposed on the combustion chamber 18.

Next, referring to FIG. 4, we discuss the fuel cell unit 16. FIG. 4 is apartial section showing a solid oxide fuel cell system (SOFC) fuel cellunit according to an embodiment of the present invention.

As shown in FIG. 4, the fuel cell unit 16 is furnished with a fuel cell84 and internal electrode terminals 86, respectively connected to therespective terminals at the top and bottom of the fuel cell 84.

The fuel cell 84 is a tubular structure extending in the verticaldirection, furnished with a cylindrical internal electrode layer 90, onthe inside of which is formed a fuel gas flow path 88, a cylindricalexternal electrode layer 92, and an electrolyte layer 94 between theinternal electrode layer 90 and the external electrode layer 92. Thisinternal electrode layer 90 is a fuel electrode through which fuel gaspasses, and is a (−) pole, while the external electrode layer 92 is anair electrode which contacts the air, and is a (+) pole.

The internal electrode terminals 86 attached at the top end and bottomends of the fuel cell device 16 have the same structure, therefore wewill here discuss specifically the internal electrode terminal 86attached at the top and side. The top portion 90 a of the insideelectrode layer 90 is furnished with an outside perimeter surface 90 band top end surface 90 c, exposed to the electrolyte layer 94 and theoutside electrode layer 92. The inside electrode terminal 86 isconnected to the outer perimeter surface of the inside electrode layer90 through a conductive seal material 96, and is electrically connectedto the inside electrode layer 19 by making direct contact with the topend surface 90 c of the inside electrode layer 90. A fuel gas flow path98 communicating with the inside electrode layer 90 fuel gas flow path88 is formed at the center portion of the inside electrode terminal 86.

The inside electrode layer 90 is formed, for example, from at least oneof a mixture of Ni and zirconia doped with at least one type of rareearth element selected from Ca, Y, Sc, or the like; or a mixture of Niand ceria doped with at least one type of rare earth element; or anymixture of Ni with lanthanum gallate doped with at least one elementselected from among Sr, Mg, Co, Fe, or Cu.

The electrolyte layer 94 is formed, for example, from at least one ofthe following: zirconia doped with at least one type of rare earthelement selected from among Y, Sc, or the like; ceria doped with atleast one type of selected rare earth element; or lanthanum gallatedoped with at least one element selected from among Sr or Mg.

The outside electrode layer 92 is formed, for example, from at least oneof the following: lanthanum manganite doped with at least one elementselected from among Sr or Ca; lanthanum ferrite doped with at least oneelement selected from among Sr, Co, Ni, or Cu; lanthanum cobaltite dopedwith at least one element selected from among Sr, Fe, Ni, or Cu; silver,or the like.

Next we discuss the fuel cell stack 14, referring to FIG. 5. FIG. 5 is aperspective view showing the fuel cell stack in a solid oxide fuel cellsystem (SOFC) according to an embodiment of the present invention.

As shown in FIG. 5, the fuel cell stack 14 is furnished with sixteenfuel cell units 16; the top inside and bottom inside of these fuel cellunits 16 are respectively supported by a lower support plate 68 andupper support plate 100. Through holes 68 a and 100 a, through which theinside electrode terminal 86 can penetrate, are provided on this lowersupport plate 68 and outer support plate 100.

In addition, a collector 102 and an external terminal 104 are attachedto the fuel cell unit 16. This collector 102 is integrally formed by afuel electrode connecting portion 102 a, which is electrically connectedto the inside electrode terminal 86 attached to the inside electrodelayer 90 serving as the fuel electrode, and by an air electrodeconnecting portion 102 b, which is electrically connected to the entireexternal perimeter of the outside electrode layer 92 serving as the airelectrode. The air electrode connecting portion 102 b is formed of avertical portion 102 c extending vertically along the surface of theoutside electrode layer 92, and multiple horizontal portions 102 dextending in the horizontal direction from this vertical portion 102 calong the surface of the outside electrode layer 92. The fuel electrodeconnecting portion 102 a extends linearly in an upward or downwarddiagonal direction from the vertical portion 102 c of the air electrodeconnecting portion 102 b toward the inside electrode terminals 86positioned in the upper and lower directions on the fuel cell unit 16.

Furthermore, electrode terminals 86 at the top and bottom ends of thetwo fuel cell units 16 positioned at the end of the fuel cell stack 14(at the front and back sides on the left edge in FIG. 5) arerespectively connected to the outside terminals 104. These externalterminals 104 are connected to the external terminals 104 (not shown) atthe ends of the adjacent fuel cell stack 14, and as described above, allof the 160 fuel cell units 16 are connected in series.

Next, referring to FIG. 6, we discuss the sensors attached to the solidoxide fuel cell system (SOFC) according to the present embodiment. FIG.6 is a block diagram showing a solid oxide fuel cell system (SOFC)according to an embodiment of the present invention.

As shown in FIG. 6, a solid oxide fuel cell system 1 is furnished with acontrol unit 110; an operating device 112 provided with operatingbuttons such as “ON” or “OFF” for user operation, a display device 114for displaying various data such as a generator output value (Watts),and a notification device 116 for issuing warnings during abnormalstates and the like are connected to this control unit 110. Thisnotification device 116 may be connected to a remote control center toinform the control center of abnormal states.

Next, signals from the various sensors described below are input to thecontrol unit 110.

First, a flammable gas detection sensor 120 detects gas leaks and isattached to the fuel cell module 2 and the auxiliary unit 4.

The purpose of the flammable gas detection sensor 120 is to detectleakage of CO in the exhaust gas, which is meant to be exhausted to theoutside via the exhaust gas conduit 80, into the external housing (notshown) which covers the fuel cell module 2 and the auxiliary unit 4.

A water reservoir state detection sensor 124 detects the temperature andamount of hot water in a water heater (not shown).

An electrical power state detection sensor 126 detects current, voltage,and the like in the inverter 54 and in a distribution panel (not shown).

A generator air flow detection sensor 128 detects the flow volume ofgenerator air supplied to the generating chamber 10.

A reforming air flow volume sensor 130 detects the volume of reformingair flow supplied to the reformer 20.

A fuel flow volume sensor 132 detects the flow volume of fuel gassupplied to the reformer 20.

A water flow volume sensor 134 detects the flow volume of pure watersupplied to the reformer 20.

A water level sensor 136 detects the water level in pure water tank 26.

A pressure sensor 138 detects pressure on the upstream side outside thereformer 20.

An exhaust temperature sensor 140 detects the temperature of exhaust gasflowing into the hot water producing device 50.

As shown in FIG. 3, a generating chamber temperature sensor 142 isdisposed on the front surface side and rear surface side around the fuelcell assembly 12, and detects the temperature around the fuel cell stack14 in order to estimate the temperature of the fuel cell stack 14 (i.e.,of the fuel cell 84 itself).

A combustion chamber temperature sensor 144 detects the temperature incombustion chamber 18.

An exhaust gas chamber temperature sensor 146 detects the temperature ofexhaust gases in the exhaust gas chamber.

A reformer temperature sensor 148 detects the temperature of thereformer 20 and calculates the reformer 20 temperature from the intakeand exit temperatures on the reformer 20.

If the solid oxide fuel cell system (SOFC) is placed outdoors, theoutside temperature sensor 150 detects the temperature of the outsideatmosphere. Sensors to detect outside atmospheric humidity and the likemay also be provided.

Signals from these various sensor types are sent to the control unit110; the control unit 110 sends control signals to the water flowregulator unit 28, the fuel flow regulator unit 38, the reforming airflow regulator unit 44, and the oxidant gas supply device 45 based ondata from the sensors, and controls the flow volumes in each of theseunits.

Next, referring to FIG. 7, we discuss the operation of a solid oxidefuel cell system (SOFC) according to the present embodiment at the timeof start up. FIG. 7 is a timing chart showing the operations of a solidoxide fuel cell system (SOFC) according to an embodiment of the presentinvention at the time of start up.

At the beginning, in order to warm up the fuel cell module 2, operationstarts in a no-load state, i.e., with the circuit which includes thefuel cell module 2 in an open state. At this point current does not flowin the circuit, therefore the fuel cell module 2 does not generateelectricity.

First, reforming air is supplied from the reforming air flow regulatorunit 44 to the reformer 2 on the fuel cell module 2. At the same time,generating air is supplied from the oxidant gas supply device 45 to thefuel cell module 2 air heat exchanger 22, and this generating airreaches the generating chamber 10 and the combustion chamber 18.

Immediately thereafter, fuel gas is also supplied from the fuel flowregulator unit 38, and fuel gas into which reform area is blended passesthrough the reformer 20, the fuel cell stack 14, and the fuel cell unit16 to reach the combustion chamber 18.

Next, ignition is brought about by the ignition device 83, and fuel gasand air (reforming air and generating air) supplied to the combustionchamber 18 is combusted. This combustion of fuel gas and air producesexhaust gas; the generating chamber 10 is warmed by this exhaust gas,and when the exhaust gas rises into the fuel cell module 2 sealed space8, the fuel gas, which includes reforming air in the reformer 20 iswarm, as is the generating air inside the air heat exchanger 22.

At this point, fuel gas into which reform area is blended is supplied tothe reformer 20 by the fuel flow regulator unit 38 at the reforming airflow regulator unit 44, therefore the partial oxidation reformingreaction POX given by Expression (1) proceeds. This partial oxidationreforming reaction POX is an exothermic reaction, and therefore hasfavorable starting characteristics. The fuel gas whose temperature hasrisen is supplied from the fuel gas supply line 64 to the bottom of thefuel cell stack 14, and by this means the fuel cell stack 14 is heatedfrom the bottom, and the combustion chamber 18 is also heated by thecombustion of the fuel gas and air, so that the fuel stack 14 is alsoheated from above, enabling as a result an essentially uniform rise intemperature in the vertical direction of the fuel cell stack 14. Eventhough the partial oxidation reforming reaction POX is progressing, theongoing combustion reaction between fuel gas and air is continued in thecombustion chamber 18.

C_(m)H_(n) +xO₂ →aCO₂ +bCO+cH₂  (1)

When the reformer temperature sensor 148 detects that the reformer 20has reached a predetermined temperature (e.g. 600° C.) after the startof the partial oxidation reforming reaction POX, a pre-blended gas offuel gas, reforming air, and steam is applied to the reformer 20 by thewater flow regulator unit 28, the fuel flow regulator unit 38, and thereforming air flow regulator unit 44. At this point an auto-thermalreforming reaction ATR, which makes use of both the aforementionedpartial oxidation reforming reaction POX and the steam reformingreaction SR described below, proceeds in the reformer 20. Thisauto-thermal reforming reaction ATR can be internally thermallybalanced, therefore the reaction proceeds in a thermally independentfashion inside the reformer 20. In other words, when there is a largeamount of oxygen (air), heat emission by the partial oxidation reformingreaction POX dominates, and when there is a large amount of steam, theendothermic steam reforming reaction SR dominates. At this stage, theinitial stage of startup has passed and some degree of elevatedtemperature has been achieved within the generating chamber 10,therefore even if the endothermic reaction is dominant, no major drop intemperature will be caused. Also, the combustion reaction continueswithin the combustion chamber 18 even as the auto-thermal reformingreaction ATR proceeds.

When the reformer temperature sensor 146 detects that the reformer 20has reached a predetermined temperature (e.g., 700° C.) following thestart of the auto-thermal reforming reaction ATR shown as Expression(2), the supply of reforming air by the reforming air flow regulatorunit 44 is stopped, and the supply of steam by the water flow regulatorunit 28 is increased. By this means, a gas containing no air and onlycontaining fuel gas and steam is supplied to the reformer 20, where thesteam reforming reaction SR of Expression (3) proceeds.

C_(m)H_(n) +xO₂ +yH₂O→aCO₂ +bCO+cH₂  (2)

C_(m)H_(n) +xH₂O→aCO₂ +bCO+cH₂  (3)

This steam reforming reaction SR is an endothermic reaction, thereforethe reaction proceeds as a thermal balance is maintained with thecombustion heat from the combustion chamber 18. At this stage, the fuelcell module is in the final stages of startup, therefore the temperaturehas risen to a sufficiently high level within the generating chamber 10so that no major temperature dropped is induced in the generatingchamber 10 even though an endothermic reaction is proceeding. Also, thecombustion reaction continues to proceed in the combustion chamber 18even as the steam reforming reaction SR is proceeding.

Thus, after the fuel cell module 2 has been ignited by the ignitiondevice 83, the temperature inside the generating chamber 10 graduallyrises as a result of the partial oxidation reforming reaction POX, theauto-thermal reforming reaction ATR, and the steam reforming reaction SRwhich proceed in that sequence. Next, when the temperature inside thegenerating chamber 10 and the temperature of the fuel cell 84 reaches apredetermined generating temperature which is lower than the ratedtemperature at which the cell module 2 can be stably operated, thecircuit which includes the fuel cell module 2 is closed, electricalgeneration by the fuel cell module 2 begins, and current then flows tothe circuit. Generation of electricity by the fuel cell module 2 causesthe fuel cell 84 itself to emit heat, such that the temperature of thefuel cell 84 rises. As a result, the rated temperature at which the fuelcell module 2 is operated becomes, for example, 600° C.-800° C.

Following this, a quantity of fuel gas and air greater than thatconsumed by the fuel cell 84 is applied in order to maintain the ratedtemperature and continue combustion inside the combustion chamber 18.Generation of electricity by the high reform-efficiency steam reformingreaction SR proceeds while electricity is being generated.

Next, referring to FIG. 8, we discuss the operation upon stopping thesolid oxide fuel cell system (SOFC) of the present embodiment. FIG. 8 isa timing chart showing the operations which occur upon stopping thesolid oxide fuel cell system (SOFC) of the present embodiment.

As shown in FIG. 8, when stopping the operation of the fuel cell module2, the fuel flow regulator unit 38 and the water flow regulator unit 28are first operated to reduce the quantity of fuel gas and steam beingsupplied to the reformer 20.

When stopping the operation of the fuel cell module 2, the quantity ofgenerating air supplied by the reforming air flow regulator unit 44 intothe fuel cell module 2 is being increased at the same time that thequantity of fuel gas and steam being supplied to the reformer 20 isbeing reduced; the fuel cell assembly 12 and the reformer 20 are aircooled to reduce their temperature. Thereafter, when the temperature ofthe generating chamber drops to, for example, 400° C., supply of thefuel gas and steam to the reformer 20 is stopped, and the steamreforming reaction SR in the reformer 20 ends. Supply of the generatingair continues until the temperature in the reformer 20 reaches apredetermined temperature, e.g. 200° C.; when the predeterminedtemperature is reached, the supply of generating air from the oxidantgas supply device 45 is stopped.

Thus in the present embodiment, the steam reforming reaction SR by thereformer 20 and cooling by generating air are used in combination,therefore when the operation of the fuel cell module 2 is stopped, thatoperation can be stopped relatively quickly.

Next, referring to the FIG. 9 through 17, we discuss the control of asolid oxide fuel cell system 1 according to a first embodiment of thepresent invention.

FIG. 9 is graph showing the relationship between output current and fuelsupply amount in the solid oxide fuel cell system 1 of the firstembodiment of the present invention.

FIG. 10 is a graph showing the relationship between output current andthe amount of heat produced by supplied fuel in the solid oxide fuelcell system 1 of the first embodiment of the present invention.

First, as shown by the solid line in FIG. 9, the solid oxide fuel cellsystem 1 of the present embodiment is capable of changing output at orbelow the rated output power of 700 W (output current 7 A) in responseto power demand. The fuel supply amount (L/min) deemed necessary tooutput a required power is set by the Basic Fuel Supply Table shown bythe solid line in FIG. 9. A control section 110, which serves ascontroller, determines a fuel supply amount based on the fuel supplyamount table in response to the power demand detected by electricalpower state detecting sensor 126, which serves as demand power detectioncircuit; the fuel flow regulator unit 38 serving as fuel supply deviceis controlled based on this.

The amount of fuel needed for generation is proportional to output power(output current), but as shown by the solid line in FIG. 9, the fuelsupply amount set in the basic fuel supply amount table is notproportional to output current. This is because when the fuel supplyamount is reduced in proportion to output power, it becomes impossibleto maintain the fuel cell units 16 in the fuel cell module 2 at atemperature capable of generating electricity. Therefore in the presentembodiment the basic fuel supply table is set at a fuel utilization rateof approximately 70% when generating a large power in the region of a 7A output current, and is set at a fuel utilization rate of approximately50% when generating a small power in the region of a 2 A output current.Thus by reducing the fuel utilization rate in the small power generationregion and using the fuel not used for generation to combust and heatthe reformer 20 and the like, temperature drops in the fuel cell units16 can be suppressed, and an electrical generation temperature can bemaintained in the fuel cell module 2.

However, reducing the fuel utilization rate causes an increase in fuelnot contributing to electrical generation, so the energy efficiency ofthe solid oxide fuel cell system 1 declines in the small powergeneration region. In the solid oxide fuel cell system 1 of the presentembodiment, a fuel table change circuit 110 a built into the controlsection 110 changes or corrects the fuel supply amount set in the basicfuel supply table in response to predetermined conditions, reducing thefuel supply amount as shown by the dotted line in FIG. 9, so that thefuel utilization rate in the small power generation region is raised.The energy efficiency of the solid oxide fuel cell system 1 is thusimproved.

FIG. 10 is a graph schematically showing the relationship between outputcurrent when fuel is supplied based on the basic fuel supply tableversus amount of heat from the supplied fuel in the solid oxide fuelcell system 1 of the present embodiment. As shown by the dot-and-dashline in FIG. 10, the amount of heat needed to make the fuel cell module2 thermally autonomous and to operate stably increases monotonicallywith the increase in output current. The solid line graph in FIG. 10shows the heat amount when fuel is supplied according to the basic fuelsupply table. In this embodiment, the necessary heat amount indicated bythe dot-and-dash line and the amount of heat supplied based on the basicfuel supply table shown by the solid line are approximately matched inthe region below an output current of 5 A, which corresponds to mediumpower generation.

Furthermore, in the region above an output current of 5 A, the heatamount shown by the solid line and supplied according to the basic fuelsupply table is greater than the heat amount shown by the dot-and-dashline, which is the minimum requirement for thermal autonomy. The surplusheat amount between the solid line and the dotted line is accumulated inthe insulating material 7 serving as heat storing material. There isalso a correlation between the output current from the solid oxide fuelcell system 1 and the temperature of the fuel cell units 16 in the fuelcell module 2 when this current is being output in a steady state; sincethe temperature of the fuel cell units 16 must be raised in order toincrease output current, the temperature of the fuel cell units 16 ishigh when in a high output current state. In the present embodiment anoutput current of 5 A corresponds to approximately 633° C., which is thestored heat temperature Th. Therefore in the solid oxide fuel cellsystem 1 of the present embodiment, a larger amount of heat isaccumulated in the insulating material 7 when the output current is 5 Aand the stored heat temperature Th=approximately 633° C. or above.

This stored heat temperature Th is set to a temperature corresponding to500 W (an output current of 5 A), which is larger than the 350 Wrepresenting the midpoint value of the generated power range of 0 W-700W. In the region of an output current of 5 A or below, the heat amountsupplied based on the basic fuel supply table is set to be approximatelythe same as the minimum required heat amount for thermal autonomy (theheat amount in the basic fuel supply table is slightly higher).Therefore as shown by the dotted line example in FIG. 10, the heatamount needed for thermal autonomy is lacking when the fuel supplyamount from the basic fuel supply table is corrected to reduce the fuelsupply amount.

In the present embodiment, as described below, a correction is made inthe small power generation region to temporarily reduce the fuel supplyamount set by the basic fuel supply table and raise the fuel utilizationrate. At the same time, the lacking heat amount caused by the reductionin the fuel supply amount from the basic fuel supply table isreplenished by using the heat amount accumulated in the insulatingmaterial 7 while the fuel cell module 2 is operating in a region abovethe stored heat temperature Th. Note that in the present embodiment,because the heat capacity of the insulating material 7 is extremelyhigh, the heat amount accumulated in the insulating material 7 can beused over a period of more than 2 hours when the fuel cell module 2 isoperating in the small generated power region after operating for apredetermined time at high generated power, and the fuel utilizationrate can be raised by performing a correction to reduce the fuel supplyamount during this interval.

Also, in the present embodiment when the output current is 5 A and thestored heat temperature Th=approximately 633° C. or above, the basicfuel supply table is set so that a greater heat amount is accumulated inthe insulating material 7, but the basic fuel supply table can also beset approximately the same as the minimum required heat amount forthermal autonomy, even in an output current region of 5 A or above. Thatis, in a region where the generated power is large, the operatingtemperature of the fuel cell module 2 is higher than when generatedpower is small, therefore even if the fuel supply amount is set for theminimum required heat amount for thermal autonomy, the usable heatamount during small power generation can be accumulated in theinsulating material 7. As in this embodiment, the required heat amountcan be reliably accumulated in the insulating material 7 during theshort evening time period when power demand is at peak by activelysetting the fuel supply amount to be high at times of high generatedpower.

Next, referring to the FIG. 11 through 17, we discuss the specificcontrol of a solid oxide fuel cell system 1 according to the firstembodiment of the present invention. FIG. 11 is a control flow chart ofthe fuel supply amount in the solid oxide fuel cell system of the firstembodiment of the present invention. FIG. 12 is a stored heat amountestimate table used to estimate the amount of heat accumulated in theinsulating material 7. FIG. 13 is a graph of the stored heat amountestimate table. FIG. 14 is a graph showing the values of first modifyingcoefficients relative to output current. FIG. 15 is a graph showing thevalues of second modifying coefficients relative to output current.

The flow chart shown in FIG. 11 is executed at predetermined timeintervals in the control section 110 during generating operations of thesolid oxide fuel cell system 1. First, as step S1 in FIG. 11,integration processing is executed based on the stored heat estimatingtable shown in FIG. 12. The integral value Ni calculated in step S1 is,as described below, a value which will serve as an index for a usablestored heat amount accumulated in the insulating material 7 or the like,and lies between 0 and 1.

Next, in step S2, a judgment is made as to whether the integral value Nicalculated in step S1 is a 0. If the integral value Ni is 0, the systemproceeds to step S3; if other than 0, it proceeds to step S4.

When the integral value Ni is a 0, it is estimated that heat sufficientto be usable has not accumulated in the insulating material 7 or thelike, therefore in step S3 the fuel supply amount is determined by thecontrol section 110 based on the basic fuel supply table. The controlsection 110 sends a signal to the fuel flow regulator unit 38, and thedetermined fuel supply amount is supplied to the fuel cell module 2.Therefore in this case no correction is executed to raise the fuelutilization rate even if generated power is small. After step S3, oneiteration of the processing in the flow chart of FIG. 11 is completed.

In step S4, on the other hand, the amount of change in the rate ofutilization versus the fuel supply amount determined by the basic fuelsupply table is determined based on the integral value Ni. That is, whenthe integral value Ni is 1, the fuel supply amount is reduced the most;the fuel utilization rate is improved, and the amount of reduction infuel supply amount decreases as the integral value Ni decreases.

Next, in step S5, a first modifying coefficient is determined based onthe graph shown in FIG. 14. As shown in FIG. 14, the first modifyingcoefficient is 1 in the small output current region, and goes to 0 whenoutput current exceeds 4.5 A. That is, in the small generated powerregion, a correction is executed to reduce the fuel supply amount byusing the heat amount accumulated in the insulating material 7, and thefuel utilization rate is improved, whereas no correction is executed inthe large generated power region. This is because in the large generatedpower region operation can be conducted at a sufficiently high the fuelutilization rate using the basic fuel supply table, as well, and whenthe generated power is large, it is difficult to utilize the stored heatin the insulating material 7 due to the high temperature inside the fuelcell module 2.

Next, in step S6, a second modifying coefficient is determined based onthe graph shown in FIG. 15. As shown in FIG. 15, the second modifyingcoefficient is 0.5 in the region where output current is 1 A or below,and grows linearly in the output current region of 1 to 1.5 A, reaching1 at an output current of 1.5 or above. In other words, in the powergeneration region of 150 W or below, which is the utilizationsuppression generation amount, the absolute value of the fuel supplyamount according to the basic fuel supply table is small, so there is arisk of damage to the fuel cell units 16 when a large correction is maderesulting in a reduced fuel supply amount. Also, by keeping the amountof correction to the basic fuel supply table low, the heat amountaccumulated in the insulating material 7 can be used a little at a time,making it possible to utilize the stored heat over a long period.Therefore the second modifying coefficient causes the amount ofcorrection to the basic fuel supply table to be reduced to the degreethat generated power is small, so that it functions as a change periodextension circuit for extending the time period for changing orcorrecting the basic fuel supply table. This change period extensioncircuit operates so that the stored heat accumulated in the insulatingmaterial 7 is used after correction to the basic fuel supply tablebegins, therefore the stored heat amount gradually decreases as thelength of time during which correction is being executed increases, andsince the amount of correction to the fuel utilization rate declineswhen the stored heat amount declines, the period over which stored heatcan be used is further extended.

Note that it is also acceptable not to modify the correction amountusing the second modifying coefficient.

Next, in step S7, the first modifying coefficient determined in step S5is multiplied by the second modifying coefficient determined in step S6to determine a final utilization change rate. Moreover, the amount ofcorrection to the water supply amount is determined according to thedetermined fuel supply amount, and the generating air supply amount isreduced 10% relative to the normal air supply amount. Also, the fuelsupply amount control gain is increased by 10% relative to the controlgain during normal operation, thereby improving followingcharacteristics when changing the fuel supply amount.

Thus by increasing the fuel supply amount control gain when executingcorrections to the basic fuel supply table and setting fuel supplyamount following characteristics to a high level, the fuel supply amountcan be quickly increased when the post-correction fuel utilization ratedeclines with the reduction in estimated stored heat amount. Excessivecooling of the fuel cell module 2 caused by delays in response to theincrease in fuel supply amount can thus be prevented. Therefore thecontrol to increase gain in step S7 acts as an overcooling preventioncircuit. By reducing the secondary generating air amount by 10%, coolingof the of the cells, reformer, etc., inside fuel cell module 2 can besuppressed, thereby enabling the reduction in stored heat amount to beinhibited and permitting effective use of stored heat. As a result,control to reduce the secondary air by 10% also acts as an overcoolingprevention circuit.

In step S8 the control section 110 sends a signal to the fuel flowregulator unit 38, the water flow regulator unit 28, and the generatingair flow regulator unit 45, and the amounts of fuel, water, andgenerating air determined in step S7 are supplied to the fuel cellmodule 2. After step S8, one iteration of the processing in the flowchart of FIG. 11 is completed. When the integral value Ni declines as aresult of executing a correction to the basic fuel supply table,processing shifts again from step S2 to step S3. Correction to the basicfuel supply table thus ends, and control of the fuel supply amount basedon the basic fuel supply table is again executed.

Next, referring to FIGS. 12 and 13, we discuss estimation of the storedheat accumulated in the insulating material 7 or the like.

Estimation of stored heat is executed by a stored heat estimatingcircuit 110 b (FIG. 6) built into the control section 110. When step S1in the flow chart shown in FIG. 11 is executed, the stored heatestimating circuit 110 b reads the temperature of the generating chamberfrom the generating chamber temperature sensor 142 serving astemperature detection device. Next, the stored heat estimating circuit110 b refers to the stored heat estimating table shown in FIG. 12 anddetermines an add/subtract value based on the generating chambertemperature sensor 142 detected temperature Td. For example, when thedetected temperature Td is 645° C., the addition value is determined at1/50,000, and this value is added to the integral value Ni. Integrationof this type is executed at a predetermined time interval after startupof the solid oxide fuel cell system 1. In this embodiment, the flowchart in FIG. 11 is executed every 0.5 seconds, therefore an integrationis executed once each 0.5 seconds. Therefore when the detectedtemperature Td is fixed at 645° C., for example, a value of 1/50,000 isintegrated once each 0.5 seconds, and the integral value Ni grows.

This integral value Ni reflects the temperature history in the fuel cellmodule 2, and within the generating chamber, etc., and becomes a valueserving as an index for showing the level of stored heat amountaccumulated in the insulating material 7 or the like. This integralvalue Ni is limited to a range of 0 to 1; when the integral value Nireaches 1, that value is held at 1 until the next subtraction occurs;when the integral value Ni has declined to 0, the value is held at 0until the next addition takes place. In the present invention, it isassumed that the value serving as index for indicating the degree of thestored heat amount is an estimated value for the stored heat amount.Therefore in the present invention the stored heat amount is estimatedbased on the temperature of the fuel cell module 2.

The utilization change amount relative to the basic fuel supply table,which is calculated in step S4 of the flow chart shown in FIG. 11, isdetermined by multiplying a predetermined correction amount times theintegral value Ni. Therefore the larger the integral value Ni serving asan estimated stored heat amount, the more the correction amountincreases; the utilization change amount is at a maximum when theintegral value Ni is a 1, and when the integral value Ni is a 0, nocorrection is executed (utilization change amount=0). That is, when theintegral value Ni is 0, the stored heat amount estimated value is judgedto be under the change-executable stored heat amount for executingcorrections to the basic fuel supply table, and no correction to thefuel utilization rate is executed.

As shown in FIGS. 12 and 13, in the present embodiment integration iscarried out as addition when the detected temperature Td is higher thanthe reference detected temperature Tcr of 635° C., and as subtractionwhen it is lower than same. That is, when the detected temperature Td ishigher than the reference temperature Tcr, an amount of heat usable forincreasing the fuel utilization rate in the insulating material 7 or thelike is accumulated, and when lower than the change referencetemperature Tcr, it is assumed that heat accumulated in the insulatingmaterial 7 or the like will be carried off, and the integral value Ni iscalculated. Put another way, the integral value Ni corresponds to thetime integral of the temperature deviation relative to the detectedtemperature Td change reference temperature Tcr, and the stored heatamount is estimated based on this integral value Ni.

Note that in this embodiment the change reference temperature Tcr whichserves as reference for estimating the stored heat amount is set to beslightly higher than the stored heat temperature Th at which there is alarge accumulation of heat (FIG. 10). For this reason the estimatedvalue of the stored heat amount is estimated to be slightly less thanactual. Therefore corrections to raise the fuel utilization rate areexecuted excessively based on the stored heat amount estimated higherthan the actual, and inducement of excessive temperature drops in thefuel cell module 2 is avoided.

Therefore a correction is made to the basic fuel supply table whengenerated power decreases in a state whereby the detected temperature Tdis higher than the change reference temperature Tcr. On the other hand,when the generated power has declined in a state whereby the detectedtemperature Td is lower than the change reference temperature Tcr, theamount of correction to the basic fuel supply table is reduced (by thedecline in integral value Ni), or the correction is not executed (whenintegral value Ni is 0).

Specifically, as shown in FIGS. 12 and 13, when the detected temperatureTd is below 580° C., there is a reduction of 20/50,000 from the integralvalue Ni. When the detected temperature Td is 580° C. or above and lessthan 620° C., there is a reduction from integral value Ni of10/50,000×(620−Td)/(620−580). When the detected temperature Td is 620°C. or above and less than 630° C., there is a reduction from integralvalue Ni of 1/50,000. Thus the integral value Ni is rapidly decreased inproportion to the degree to which the detected temperature Td is lessthan change reference temperature Tcr, and with this the amount ofcorrection to the fuel utilization rate is also rapidly reduced.

On the other hand, when the detected temperature Td is 650° C. or above,1/50,000×(Td−650) is added to the integral value Ni. When the detectedtemperature Td is 640° C. or above and less than 650° C., 1/50,000 isadded to the integral value Ni. Thus the integral value Ni is rapidlyincreased in proportion to the degree to which the detected temperatureTd is more than change reference temperature Tcr, and with this theamount of correction to the fuel utilization rate is also rapidlyincreased.

Furthermore, between detected temperatures Td of 630° C. and 640° C.,processing will differ between cases where the detected temperature Tdis tending to increase versus those in which it is tending to decrease.

That is, when the detected temperature Td is between 630° C. and 632°C., an addition value of 0 is adopted (no add/subtract is performed)when the detected temperature Td is in a rising trend, whereas 1/50,000is subtracted when it is in a declining trend. Thus when the detectedtemperature Td is less than the change reference temperature Tcr, andthe difference between them is a minor deviation temperature of 5° C. orbelow, integral value Ni is more rapidly decreased when the detectedtemperature Td is on a decreasing trend than when it is on an increasingtrend. Here, when the insulating material 7 or the like has an extremelyhigh heat capacity, and the detected temperature Td has for the momententered a decreasing trend, it can be anticipated that the temperaturewill continue to drop for a certain period of time. Therefore in suchcircumstances it is necessary to avoid the risk of a major temperaturedrop in the fuel cell module 2 by quickly reducing the integral value Niand suppressing corrections raising the fuel utilization rate (reducingthe fuel supply amount).

On the other hand, when the detected temperature Td is between 638° C.and 640° C., 1/50,000 is added when detected temperature Td is on anincreasing trend, whereas an addition value of 0 is adopted (noadd/subtract is performed) when it is in a declining trend. As describedabove, when the insulating material 7 or the like has an extremely highheat capacity, and the detected temperature Td has for the momententered an increasing trend, it can be anticipated that the temperaturewill continue to rise for a certain period of time. Therefore in suchcircumstances stored heat is actively utilized to improve the fuelutilization rate by promoting correction to raise the fuel utilizationrate (reduce the fuel supply amount) by quickly increasing the integralvalue Ni.

Different values for the add/subtract value relative to the integralvalue Ni are thus adopted in accordance with the state of change in thedetected temperature Td. Therefore the relationship between thetemperature deviation between the detected temperature Td and the changereference temperature Tcr, and the integral value Ni reflecting thestored heat amount, is changed in response to the state of change in thedetected temperature Td.

Also, when the detected temperature Td is 632° C. or above and less than638° C., the detected temperature Td is close to the change referencetemperature Tcr of 635° C. and is deemed to be stable; 0 is used as theadded value regardless of the detected temperature Td trend, and thecurrent status is maintained.

Next, referring to FIG. 16, we explain processing when the fuel cellmodule 2 has degraded. FIG. 16 is a flowchart for changing correctionamounts when the fuel cell module 2 has degraded.

When degradation of the fuel cell units 16 advances due to long years ofuse, the power extractable from a given fuel supply amount declines. Inconjunction with this, the temperature of the fuel cell units 16 alsorises for the same power produced. In the solid oxide fuel cell system 1of the present embodiment, a determination of degradation of the fuelcell module 2 (the fuel cell units 16) is made based on the temperatureof the fuel cell module 2 at a predetermined electrical generating time.Note that degradation of a fuel cell module can also be determined fromthe power or voltage, etc., which can be extracted from a predeterminedfuel supply amount.

The flow chart shown in FIG. 16 is executed each predeterminedperiod—for example several months to several years—by the controlsection 110. First, in step S21 of FIG. 16, a judgment is made as towhether the fuel cell units 16 have degraded. If it is determined thatthe fuel cell units 16 have not degraded, the processing of oneiteration of the flow chart shown in FIG. 16 ends. If it is determinedthat the fuel cell units 16 have degraded, the system advances to stepS22.

In step S22 the change reference temperature Tcr is changed to a value5° C. higher, and the third modifying coefficient is set to 0.8,completing the processing of one iteration of the flowchart shown inFIG. 16. This change is made to match the temperature which serves asthe reference for the fuel utilization rate correction, since when thefuel cell units 16 degrades, the operating temperature of the fuel cellmodule 2 as a whole shifts to the higher temperature side. The thirdmodifying coefficient is multiplied times the amount of change in theutilization rate determined in step S4 of FIG. 11. A third modifyingcoefficient is set prior to the degradation of the fuel cell units 16,and when it is determined that degradation has occurred, it is changedto 0.8, and the utilization rate change amount is reduced by 20%.Promotion of degradation of the fuel cell units 16 caused by largecorrections to the fuel utilization rate with the fuel cell units 16 ina degraded state is thus prevented. Note that once it is determined thatthe fuel cell module 2 has degraded, the threshold value for thetemperature used to determine degradation is thereafter changed whendetermining the further advancement of degradation. It is thereforepossible to determine the degree of advancement of degradation over anumber of iterations. The value of the change reference temperature Teris also changed each time a judgment of degradation occurs.

Next, referring to the FIG. 17, we discuss the operation of a solidoxide fuel cell system 1 according to a first embodiment of the presentinvention. FIG. 17( a) is a diagram conceptually showing the behavior ofa solid oxide fuel cell system 1 according to the present embodiment;(b) schematically shows changes in power demand over a day in a typicalresidence. The upper graph in FIG. 17( a) conceptually illustrates thebehavior when there is not a usable amount of heat accumulated in theinsulating material 7; the middle and lower graphs respectively show thecases for small and large accumulated heat. When, as in the upperportion of FIG. 17( a), the time duration of operation with a large fuelsupply amount is short, a usable amount of heat is not accumulated inthe insulating material 7, therefore operation after generated power hasdeclined is determined based on the basic fuel supply table, and anincrease in the fuel utilization rate will not occur. When, on the otherhand, a high power generation operation continues for a certain amountof time as in the middle portion of FIG. 17( a), operation subsequent tothe reduction of generated power is carried out by utilizing the heatamount regulated in the insulating material 7 when power generation waslarge, therefore high-efficiency operation with a fuel supply amountreduced from that of the basic fuel supply table is carried out duringthe period that a usable amount of heat remains in the insulatingmaterial 7. Thus fuel corresponding to the shaded portion of the middlegraph is saved. Furthermore, when a large generated power operation iscarried out for a long period of time as shown in the bottom portion ofFIG. 17( a), a large amount of heat is accumulated in the insulatingmaterial 7, therefore high-efficiency operation utilizing accumulatedheat is carried out over a longer time, and even more fuel is saved.

Next, in FIG. 17( b) the power demand used in a residence is shown bythe solid line, the power generated by the solid oxide fuel cell system1 is shown by the dotted line, and the integral value Ni serving as anindex of the stored heat amount is shown by the dot and dash line.

First, at time t0-t1 when household members are asleep, the demandedpower used in the residence is small; at time t1 the occupants awake andthe power demand increases. In conjunction with this, the generatedpower from the solid oxide fuel cell system 1 also increases, and thatportion of the generated power exceeding the rated power of the fuelcell is supplied from the power grid. Since the state in which power useis small continues for approximately 6 to 8 hours during which theoccupants are asleep, the accumulated heat (integral value Ni) estimatedby the stored heat estimating circuit 110 b is 0 or an extremely smallvalue.

When, at time t1, generated power increases and the fuel cell module 2is operating at a temperature higher than the stored heat temperatureTh, the stored heat amount gradually increases, and at time t2 increasesto approximately 1, which is the maximum integral value. Thereafter,power demand suddenly drops when occupants leave the house at time t3.Thus when generated power drops in a state whereby the stored heatamount is equal to or greater than the change execution stored heatamount, a correction to the basic fuel supply table by the fuel tablechange circuit 110 a is executed, and the fuel utilization rate at lowgenerated power is increased. When operated at a raised the fuelutilization rate, the heat amount accumulated in the insulating material7 is utilized, therefore the integral value Ni also declines. In theembodiment, operation at an improved fuel utilization rate can becarried out for approximately 1 to 3 hours.

Next, when occupants return home at time t4, the power demand againincreases. The integral value Ni increases with some delay (time t4-t5)after the increase in power demand at time t4 and again reaches themaximum value. Next, at time t6 the occupants retire, and an increasedfuel utilization rate operation is conducted after the power demand hasdecreased (time t6 and later).

When the power demand at a residence changes in this manner, operationat an increased fuel utilization rate, in which the heat amount storedin the insulating material 7 is utilized, is carried out twice a day.This period of operation at heightened fuel utilization rate reaches asmuch as 20-50% of the small generated power period, and improves theoverall energy efficiency of the solid oxide fuel cell system 1.

In conventional solid oxide fuel cell system, when generated power issmall, generating heat drops, resulting in a tendency for the fuel cellmodule temperature to drop. The fuel utilization rate is thereforereduced at times of low generated power, and fuel not used forelectrical generation heats the fuel cell module to prevent excessivetemperature drops. In particular, in solid oxide fuel cell system oftype in which the reformer is disposed within the fuel cell module,endothermic reactions occur inside the reformer, increasing the tendencytoward temperature drops.

In the solid oxide fuel cell system 1 of the present invention, whengenerated power is small, if it is estimated by the stored heatestimating circuit 110 b that a usable amount of heat has accumulated inthe insulating material 7, the basic fuel supply table is temporarilycorrected so that the fuel utilization rate increases (FIG. 11, stepS7). Thermal autonomy of the solid oxide fuel cell system 1 is thusmaintained and overall energy efficiency of the solid oxide fuel cellsystem 1 is improved, while excessive temperature drops are avoided.

Using the solid oxide fuel cell system 1 of the present embodiment,settings are made (FIG. 10) so that more heat is accumulated in theinsulating material 7 in a temperature region above a predeterminedstored heat temperature Th, therefore accumulated heat can beeffectively utilized by actively accumulating heat in a region higherthan the stored heat temperature Th at which the fuel utilization ratecan be raised, consuming this heat at times of small power generationwhen the fuel cell module 2 temperature is comparatively low and storedheat is easy to utilize, thus effectively using the stored heat.

Using the solid oxide fuel cell system 1 of the present embodiment, thedetected temperature Td detected by the generating chamber temperaturesensor 142 reflects the heat amount stored in the insulating material 7,therefore the basic fuel supply table can be easily corrected using therelationship between detected temperature Td and change referencetemperature Tcr.

Using the solid oxide fuel cell system 1 of the present embodiment, thechange reference temperature Tcr is set higher than the stored heattemperature Th (FIG. 10), therefore stored heat will be used at or abovethe change reference temperature Tcr, which is higher than the storedheat temperature Th at which there is abundant heat stored in theinsulating material 7, so the risk can be avoided of stored heat beingused in a low stored heat amount state, causing an excessive temperaturedrop.

Using the solid oxide fuel cell system 1 of the present embodiment, thestored heat estimating circuit 110 b estimates the stored heat amountbased on the history of the detected temperature Td (FIG. 11, step S4,FIG. 13), therefore a more accurate estimate can be made compared toestimation of the stored heat amount using the current detectedtemperature Td alone, and stored heat can be more effectively used.

Using the solid oxide fuel cell system 1 of the present embodiment, thestored heat amount accumulated in the insulating material 7 is estimatedby integrating temperature deviations over time (FIG. 11, step S4, FIG.13); when the time of operation at a temperature higher than the storedheat temperature Th is long, the estimated stored heat amount is large;when that time is short, the estimated stored heat amount is small, anda more accurate estimation of the stored heat amount can be achieved.The risk of excessive temperature drops and the like due to utilizationof stored heat can thus be reliably avoided.

Using the solid oxide fuel cell system 1 of the present embodiment, theamount of correction for raising the fuel utilization rate is increasedas the stored heat amount increases (FIG. 11, step S4, FIG. 13),therefore correction can be performed to greatly improve the fuelutilization rate while reliably avoiding the risk of excessivetemperature drops and the like.

Using the solid oxide fuel cell system 1 of the present embodiment, thecorrection amount is suddenly increased to the extent the detectedtemperature Td is high, while the correction amount suddenly decreasesto the extent the detected temperature Td is low (FIG. 13), therefore asignificant fuel utilization rate correction can be made when thedetected temperature Td is high, and the compensation amount can berapidly decreased when the detected temperature Td is low, so excessivetemperature drops can be reliably prevented.

Using the solid oxide fuel cell system 1 of the present embodiment, therelationship between the estimated stored heat amount and thecompensation amount are changed in response to the state of the detectedtemperature Td or generated power (FIG. 13, 630-640° C.; FIGS. 14, 15,18), therefore the two goals of preventing excessive temperature dropsand effectively utilizing stored heat can be met.

Using the solid oxide fuel cell system 1 of the present embodiment, thefuel table change circuit 110 a reduces the compensation amount when thegenerated power is small (FIG. 15), therefore the amount of stored heatused declines, and the period during which stored heat can be used canbe extended.

Using the solid oxide fuel cell system 1 of the present embodiment, theestimated value of the stored heat amount drops suddenly when thedetected temperature Td is in a declining trend and the differencebetween the detected temperature Td and the change reference temperatureTcr is at or below a predetermined very small deviation temperature(FIG. 13, 630-632° C.), therefore the estimated value for the storedheat amount is rapidly reduced when the detected temperature Td is in adeclining phase, and excessive temperature drops can be reliablyprevented.

Using the solid oxide fuel cell system 1 of the present embodiment, thecompensation amount for increasing the fuel utilization rate is changedin response to the state of the fuel cell module 2 (FIGS. 14, 15, 16),therefore corrections to the fuel utilization rate not conforming to thestate of the fuel cell module 2 can be prevented.

Using the solid oxide fuel cell system 1 of the present embodiment, thechange reference temperature Tcr is changed to a high value when thefuel cell module 2 degrades (FIG. 16), therefore the fuel utilizationrate can be corrected without placing an excessive burden on the fuelcell module 2 when it has degraded and its operating temperature hasrisen.

Using the solid oxide fuel cell system 1 of the present embodiment, thecompensation amount is reduced when the fuel cell module 2 degrades(FIG. 16, step S22), therefore promotion of degradation can besuppressed by correcting the fuel utilization rate.

Also, in the above-described first embodiment of the present invention,the calculated subtraction or addition value to the integral value Niwas being calculated based on only the detected temperature Td shown inthe stored heat amount estimation table shown in FIG. 12, however as avariant example, the add/subtract value can also be determined by addingin output current. For example, the integral value Ni can be calculatedby integrating the value obtained by multiplying the add/subtract valuedetermined based on the FIG. 12 stored heat amount estimate table timesthe current correction coefficient shown in FIG. 18. As shown in FIG.18, the current correction coefficient is determined as 1/7 for anoutput current of 3 A or below and at 1/12 for 4 A or above, and islinearly decreased from 1/7 to 1/12 between 3 and 4 A.

By multiplying by the current correction coefficient set in this manner,the integral value Ni drops suddenly in the small generated powerregion, whereas increases and decreases in the integral value Ni in themedium generated power and greater region become gradual. Therefore bycorrecting the basic fuel supply table, the integral value Ni isgradually decreased during small power generation, which consumes largeamounts of heat accumulated in the insulating material 7. The risk ofinducing extraordinary temperature drops by overestimating the storedheat amount can thus be reliably avoided.

In the above-described embodiment the add/subtract value for addition orsubtraction to the integral value Ni was determined by the detectedtemperature Td alone, as shown in FIG. 13, but the present invention canalso be configured so that the add/subtract value is also dependent onoutput current. For example, at an output current of 3 A or below (anoutput power of 300 W), the change reference temperature Tcr can beraised about 2° C. and the entire FIG. 13 graph shifted about 2° C. Inthis manner the change reference temperature Tcr is changed to a highvalue when generated power is small, and the estimated stored heatamount is calculated as a small value. The compensation amount forincreasing the fuel utilization rate is thus reduced, therefore the fuelutilization rate is greatly improved in the region where generated poweris small and the absolute fuel supply amount is low, so that excessivedrops in the fuel supply amount can be suppressed.

Next, referring to the FIG. 19 through 33, we discuss the operation of asolid oxide fuel cell system according to a second embodiment of thepresent invention.

In the solid oxide fuel cell system of the present embodiment, controlby the control section 110 is different from that described above forthe first embodiment. Therefore here we explain only the portions of thesecond embodiment of the invention which differ from the firstembodiment, and we omit explanation of similar constitutions,operations, and effects.

In the above-described first embodiment the fuel supply amount wasdetermined based on the basic fuel supply table in response to powerdemand, temporarily changing the determined fuel supply amount so thatwas reduced based on the heat amount accumulated in the insulatingmaterial 7, temporarily increasing the fuel utilization rate. Thus inthe solid oxide fuel cell system of the second embodiment, no processingis conducted to determine the fuel supply amount based on the basic fuelsupply table and change the fuel supply amount based on an estimatedstored heat amount; rather, the fuel supply amount is directlycalculated based on a detected temperature Td or the like. In thepresent embodiment, however, the fuel supply amount, directly determinedbased on the detected temperature Td or the like, includes the additionof the heat amount accumulated in the insulating material 7, etc., andthe fuel utilization rate is improved by utilizing the stored heat inthe state in which the stored heat amount is large, therefore the sametechnical concept as in the first embodiment can be achieved.

Next, in the above-described first embodiment the change to the fuelsupply amount to increase the fuel utilization rate based on theestimated stored heat amount was accomplished by multiplying the changeamount by a first correction coefficient (FIG. 11 step S5, FIG. 14), soit was primarily used when generated power was small (at or above agenerated power of 4.5 A; first correction coefficient=0). In contrast,in the present embodiment no use is made of a coefficient correspondingto the first correction coefficient in the first embodiment. Thereforein the present embodiment, high efficiency control utilizing the heatamount accumulated in insulating material 7 is executed not only in thelow generated power region, but also in the high generated power region.Therefore in the solid oxide fuel cell system of the present embodiment,not only the effect of improving the fuel utilization rate using storedheat, but also the effect of consuming the heat amount accumulated inthe insulating material 7, and thereby suppressing temperature rises, isobtained when the fuel cell module 2 rises excessively. Note that in theabove-described first embodiment, as well, the first modifyingcoefficient is omitted (the change amount is not multiplied by the firstmodifying coefficient), thereby providing the same effect.

FIG. 19 is a graph schematically showing the relationship betweenchanges in power demand, fuel supply amount, and current actuallyextracted from a fuel cell module 2. FIG. 20 is a graph showing anexample of the relationship between generating air supply amount, watersupply amount, fuel supply amount, and current actually extracted from afuel cell module 2.

As shown in FIG. 19, the fuel cell module 2 is controlled to producepower in accordance with the power demand shown in FIG. 19( i). Based onpower demand, the control section 110 sets the fuel supply current valueIf, which is the target current to be produced by the fuel cell module2, as shown in FIG. 19( ii). The fuel supply current value If is set toroughly follow changes in power demand, but since the speed of responseby the fuel cell module 2 relative to changes in power demand isextremely sluggish, it is set to follow power demand gradually, and doesnot follow short cycle sudden changes in power demand. When power demandexceeds the maximum rated power of the solid oxide fuel cell system, thefuel supply current value If follows up to a current value correspondingto the maximum rated power, and does not get set to current values abovethat.

The control section 110 controls the fuel flow regulator unit 38 servingas fuel supply device in the manner shown in the FIG. 19( iii) graph,supplying the fuel cell module 2 with a fuel supply amount at a flowvolume capable of producing power corresponding to the fuel supplycurrent value If. Note that with a fixed fuel utilization rate, which isthe fraction of the fuel supply amount actually used to generateelectricity, the fuel supply current value If and the fuel supplycurrent value Fr are proportional. In FIG. 19, the fuel supply currentvalue If and fuel supply current value Fr are drawn as beingproportional, but as described below, in actuality the fuel utilizationrate is not fixed in this embodiment either.

Moreover, as shown in FIG. 19( iv), the control section 110 outputs asignal to the inverter 54 to output extractable current Iinv, which isthe current value which can be extracted from the fuel cell module 2.The inverter 54 extracts current (power) from the fuel cell module 2 inthe range of extractable current Iinv in response to the power demand,which changes rapidly from moment to moment. The portion of power demandexceeding the extractable current Iinv is supplied from the power grid.Here, as shown in FIG. 19, the extractable current Iinv instructed tothe inverter 54 by the control section 110 is set to change at apredetermined time delay relative to changes in the fuel supply currentvalue Fr when current is in a rising trend. For example, at time t10 inFIG. 19, the extractable current Iinv begins to increase at a delayafter fuel supply current value If and fuel supply current value Frstart to rise. At time t12, as well, the extractable current Iinv startsto increase at a delay after the fuel supply current value If and fuelsupply current value Fr increase. Thus delaying the timing at whichpower actually extracted from the fuel cell module 2 is increasedfollowing an increase in fuel supply current value Fr makes it ispossible to deal with the time delay which occurs as fuel supplied tothe fuel cell module 2 passes through the reformer 20, etc., to reachthe individual fuel cell stack 14, and the time delay until theelectrical generation reaction is actually possible after fuel reachesthe individual fuel cell stack 14, and so forth. Therefore theoccurrence of fuel cut-off in each of the fuel cell units 16 and theresulting damage to the fuel cell units 16 can be reliably prevented.

FIG. 20 shows in more detail the relationship between changes in thegenerating air supply amount, water supply amount, and fuel supplyamount versus the extractable current Iinv. Note that the graphs of thegenerating air supply amount, water supply amount, and fuel supplyamount shown in FIG. 20 are in each case converted to the current valuescorresponding to those supply amounts. In other words, assuming thesupplied generating air, water, and fuel are all set to the supplyamounts used for generating electricity without remainder, then each ofthe supply amount graphs is converted to overlap with the graph ofextractable current Iinv. Therefore the amount of mismatch between eachof the supply amount graphs and extractable current Iinv corresponds tothe surplus portion in each of the supply amounts. The residual fuelremaining without being used for generating electricity is burned in thecombustion chamber 18, which is the combustion section above theindividual fuel cell stack 14, and is used to heat the interior of thefuel cell module 2.

As shown in FIG. 20, the generating air supply amount, water supplyamount, and fuel supply amount are always above the extractable currentTiny; current exceeding the current producible using the respectivesupply amounts is extracted from the fuel cell module 2, preventingdamage to the fuel cell units 16 by fuel cut-off, air cut-off, and thelike. With respect to the fuel supply amounts supplied in excess ofextractable current Iinv, the water supply amount is set to the supplyamount at which all of the supplied fuel can be steam reformed. That is,in order that all the supplied fuel be steam-reformed, the water supplyamount is set with consideration for the ratio S/C between the amount ofsteam needed for steam reforming and the amount of carbon contained inthe fuel. Carbon deposition inside the reformer is thus prevented. Inthe FIG. 20 regions A and C, in which extractable current Iinv is on anincreasing trend associated with the increase in power demand, theamount of margin in the fuel supply amount, etc., is set to be higherthan in the B region, where extractable current Iinv is flat. Whengenerated power is increased, the fuel supply amount supplied to thefuel cell module 2 is increased by a power extraction delay circuit 110c (FIG. 6) built into the control section 110, then after a delay thegenerated power output from the fuel cell module 2 is increased. Thatis, the power actually output from the fuel cell module 2 at a delay ischanged after the fuel supply amount is changed in response to changesin power demand. In addition, when the extractable current Iinv issuddenly reduced in response to a drop in power demand (region C,beginning of region D), each supply amount is reduced after apredetermined delay time to a level below the extractable current Iinv.Therefore an extremely large amount of residual fuel occurs after asudden reduction in extractable current Iinv. Thus in cases where thepower demand suddenly drops, sudden reductions in extractable currentIinv of this type are implemented to prevent reverse current flow. Thuswhen increasing generated power and when decreasing generated power,more residual power is produced than when generated power is fixed, andthis residual power is used to heat the fuel cell module 2. Therefore itis not only in cases where the fuel cell module 2 has been operated forlong hours at high generated power, but also when generated power isfrequently increased and decreased that the fuel cell module 2 isstrongly heated, and a large amount of heat is accumulated in theinsulating material 7.

In the solid oxide fuel cell system of the present embodiment, it is notonly when generated power has declined after long operation at highgenerated power that stored heat is utilized; the heat amount beingaccumulated by increasing and decreasing generated power, etc., issuccessively utilized in response to conditions.

Next, referring to FIG. 21 through 28, we discuss a procedure fordetermining the generating air supply amount, water supply amount, andfuel supply amount based on the detected temperature Td.

FIG. 21 is a flowchart showing the order in which generating air supplyamount, water supply amount, and fuel supply amount are determined basedon detected temperature Td. FIG. 22 is a graph showing appropriate fuelcell stack 14 temperature versus generating current. FIG. 23 is a graphshowing the fuel utilization rate determined according to integralvalue. FIG. 24 is a graph showing the range of the fuel utilizationrates which can be determined relative to each generating current. FIG.25 is a graph showing air utilization rate determined according tointegral value. FIG. 26 is a graph showing the range of air utilizationrates which can be determined relative to each generating current. FIG.27 is a graph for determining water supply amounts versus a determinedair supply utilization rate. FIG. 28 is a graph showing appropriate fuelcell module 2 generating voltage versus generating current.

As shown by the dot-and-dash line in FIG. 22, in this embodiment, anappropriate temperature Ts(I) for the individual fuel cell stack 14 isdefined relative to the current to be produced by the fuel cell module2. The control section 110 controls the fuel supply amount, etc., sothat the temperature of the individual fuel cell stack 14 approaches theappropriate temperature Ts(I). Broadly speaking, that is, when thetemperature of an individual fuel cell stack 14 is high relative to thegenerating current generating current (when the individual fuel cellstack 14 temperature is above the dot-and-dash line in FIG. 22), thefuel utilization rate is increased, the heat amount accumulated in theinsulating material 7, etc., is actively consumed, and the temperatureinside the fuel cell module 2 is reduced. Conversely, when thetemperature of the individual fuel cell stack 14 is low relative to thegenerating current, the fuel utilization rate is reduced, and thetemperature inside the fuel cell module 2 is prevented from dropping.Specifically, the fuel utilization rate is not determined simply basedon the detected temperature Td alone; the fuel utilization rate isdetermined by calculating an amount reflecting stored heat byintegrating an add/subtract value determined based on detectedtemperature Td, etc. The estimation value for the stored heat amountfrom this integration of add/subtract values is calculated by a storedheat estimating circuit 101 b incorporated in the control section.

The flow chart shown in FIG. 21 determines the generating air supplyamount, water supply amount, and fuel supply amount based on thedetected temperature Td detected by the generating chamber temperaturesensor 142 serving as temperature detection device, and is executed at apredetermined time interval.

First, in step S31 of FIG. 21, an first add/subtract value M1 iscalculated based on the detected temperature Td and on FIG. 22. If thedetected temperature Td is in a predetermined temperature range relativeto the appropriate temperature Ts(I) (between the two solid lines inFIG. 22), the first add/subtract value M1 is set to 0. That is, when thedetected temperature Td is within the range

Ts(I)−Te≦Td≦Ts(I)+Te,

the first add/subtract value M1 is set to 0. Here Te is the firstadd/subtract threshold temperature. Note that in the present embodiment,the first add/subtract threshold temperature Te is 3° C.

When the detected temperature Td is below the appropriate temperatureTs(I):

Td<Ts(I)−Te  (4)

(below the bottom solid line in FIG. 22), the first add/subtract valueM1 is calculated by:

M1=Ki×(Td−(Ts(I)−Te)  (5)

At this point, first add/subtract value M1 is a negative value (asubtracting value). Note that Ki is a predetermined proportionalconstant.

When the detected temperature Td is above the appropriate temperatureTs(I):

Td>Ts(I)−Te  (6)

(above the bottom solid line in FIG. 22), first add/subtract value M1 iscalculated by:

M1=Ki×(Td−(Ts(I)+Te)  (7)

At this point, first add/subtract value M1 is a positive value (anadditive value). Thus first add/subtract value is determined based ongenerating current in addition to detected temperature Td, and thestored heat amount is estimated by integrating this. The appropriatetemperature Ts(I), in other words, is set to differ according togenerating current (power), and first add/subtract value M1 isdetermined to be a positive or negative value based on the value of(Ts(I)+Te) determined based on this appropriate temperature Ts(I), andon the value of (Ts(I)−Te).

Note that when the detected temperature Td exceeds (Ts(I)+Te), the firstadd/subtract value M1 becomes a positive value, and a change in fuelsupply amount is carried out to raise the fuel utilization rate asdescribed below, therefore in this Specification the temperature(Ts(I)+Te) relative to each generated power is referred to as the fuelutilization rate change temperature. By moving to high efficiencycontrol with increased fuel utilization rate by exceeding the fuelutilization rate change temperature (Ts(I)+Te), the timing forrestoration from high efficiency control to the target temperatureregion control at which the accumulated heat amount is not consumed is,as described below, the point at which first integral values N1 id suchas first addition/subtraction value M1 or the like decrease to 0.Therefore even after the detected temperature Td has dropped below thefuel utilization rate change temperature (Ts(I)+Te), the integral valuefirst integral value N1 id is maintained for a short time at a valuegreater than 0, and high efficiency control is implemented. Thereforethe target temperature region control restore temperature when restoredfrom high efficiency control to target temperature region control islower than the fuel utilization rate change temperature.

Next, in step S32 of FIG. 21, a second add/subtract value M2 iscalculated based on the latest detected temperature Td and the detectedtemperature Tdb detected one minute earlier. First, when the absolutevalue of the difference between the latest detected temperature Td andthe detected temperature Tdb one minute prior is less than the secondadd/subtract value threshold value temperature, the second add/subtractvalue M2 is set to 0. Note that in the present embodiment, the secondadd/subtract threshold temperature is 1° C.

When the change temperature difference, which is the difference betweenthe latest detected temperature Td and the detected temperature Tdb oneminute prior, is equal to or greater than the second add/subtract valuethreshold value temperature, the second add/subtract value M2 iscalculated as:

M2=Kd×(Td−Tdb)  (8)

This second add/subtract value M2 is a positive value (additive value)when the temperature drops is in a rising trend, and a negative value(subtractive value) when the detected temperature Td is in a fallingtrend. Note that Kd is a predetermined proportional constant. Thereforein cases where the detected temperature Td is rising, in the regionwhere the change temperature difference (Td−Tdb) is large the secondadd/subtract value M2, which is a quick response estimate value, is moresignificantly increased than in the region where the change temperaturedifference is small. Conversely, in cases where the detected temperatureis falling in the region where the absolute value of the changetemperature difference (Td−Tdb) is large, the second add/subtract valueM2 is more significantly decreased than in the region where the absolutevalue of the change temperature difference is small.

Note than in the present embodiment the proportional constant Kd is afixed value, but as a variant example, different proportional constantsKd could be used for the case where the change temperature difference ispositive and the case where it is negative. For example, theproportional constant Kd can also be set high when the changetemperature difference is negative. Thus the quick response estimatevalue is changed suddenly relative to the change temperature differencemore when the detected temperature is falling then when the detectedtemperature is rising. As a variant example, the proportional constantKd can also be set higher in the region where the absolute value of thechange temperature difference is high than the region where it is low.This results in the quick response estimate value being more suddenlychanged relative to the change temperature difference in the regionwhere the absolute value of the change temperature difference is highthan in the region where the absolute value of the change temperaturedifference is low. It is also possible to combine the change inproportional constant Kd based on whether the change temperaturedifference is positive or negative, with the change in proportionalconstant Kd based on the size of the absolute value of the changetemperature difference.

Next, in step S33 of FIG. 21, the first add/subtract value M1 calculatedin step S31 and the second add/subtract value M2 calculated in step S32are added to the first integral value N1 id. In the first integral valueN1 id, the usable stored heat amount accumulated in the insulatingmaterial 7 and the like is reflected by first add/subtract value M1, andrecent changes in detected temperature Td are reflected by secondadd/subtract value M2. In other words, the first integral value N1 idcan be used as an estimated value of usable stored heat amountaccumulated in insulating material 7 and the like. Integration occurs ina continuous manner after the start of operation of the solid oxide fuelcell system each time the FIG. 21 flow chart is executed; the firstadd/subtract value M1 and the second add/subtract value M2 are added orsubtracted to the previously calculated first integral value N1 id, andfirst integral value N1 id is updated to a new value. The first integralvalue N1 id is limited to a range of values between 0 and 4; when thefirst integral value N1 id reaches 4, the value is held at 4 until thenext subtraction occurs; when the first integral value N1 id hasdeclined to 0, the value is held at 0 until the next addition takesplace.

Note that in step S33, the value of a second integral value N2 id isalso calculated, in addition to the first integral value N1 id. Asdescribed below, the second integral value N2 id is calculated inexactly the same way as the first integral value N1 id until degradationoccurs in the fuel cell module 2, and the same value as for firstintegral value N1 id is taken.

Note that in this embodiment, as described above, an integral value iscalculated by adding the sum of first add/subtract value M1 and secondadd/subtract value M2 to the first integral value N1 id. That is, thefirst integral value N1 id is calculated using:

N1id=N1id+M1+M2  (9)

As a variant example, an integral value can also be calculated by addingthe product of the first add/subtract value M1 and the secondadd/subtract value M2. That is, in this variant example, first integralvalue N1 id is calculated using:

N1id=N1id+Km×M1×M2  (10)

Here Km is a variable coefficient which is changed in response topredetermined conditions. In this variant example, when the absolutevalue of the difference between the latest detected temperature Td andthe detected temperature Tdb one minute prior is less than the secondadd/subtract value threshold value temperature, the second add/subtractvalue M2 is set to 1.

Furthermore, in step S34 of FIG. 21, the fuel utilization rate isdetermined using the graphs in FIGS. 23 and 24, based on the calculatedfirst integral value N1 id.

FIG. 23 is a graph showing the setting value for the fuel utilizationrate Uf relative to the calculated first integral value N1 id. As shownby FIG. 23, when the first integral value N1 id is 0, the fuelutilization rate Uf is set to the minimum value fuel utilization rateUfmin. The fuel utilization rate Uf also increases with the increase infirst integral value N1 id, and at first integral value N1 id=1, becomesthe maximum value the fuel utilization rate Ufmax. During this intervalthe slope of the fuel utilization rate Uf is small in the region wherefirst integral value N1 id is small, and the slope increases as firstintegral value N1 id approaches 1. In other words, the fuel utilizationrate Uf is changed much more relative to change in the stored heatamount in the region where the estimated stored heat amount is largethan in the region where the stored heat amount is small. That is, thefuel supply amount is reduced so as to greatly increase the fuelutilization rate Uf as the estimated stored heat amount increases.Furthermore, when the first integral value N1 id is greater than 1, thefuel utilization rate Uf is fixed at maximum fuel utilization rateUfmax. The specific values of these minimum fuel utilization rate Ufminand maximum fuel utilization rate Ufmax are determined using the graphshown in FIG. 24, based on generating current. Thus when it is estimatedthat a usable amount of heat is accumulated in the heat storingmaterial, the fuel supply amount is reduced so that the fuel utilizationrate relative to the same generated power is higher than for the casewhen a usable amount of heat has not accumulated.

FIG. 24 is a graph showing the range of values obtainable for the fuelutilization rate Uf relative to each generating current; maximum andminimum values for the fuel utilization rate Uf are shown for eachgenerating current. As shown in FIG. 24, the minimum fuel utilizationrate Ufmin for each generating current is set to increase as generatingcurrent increases. That is, the setting is made so that the fuelutilization rate is high when generated power is large, and the fuelutilization rate is low when generated power is small. This minimum fuelutilization rate Ufmin straight line corresponds to the basic fuelsupply table in FIG. 9 of the first embodiment; when set to a fuelutilization rate on this straight line, the fuel cell module 2 can bethermally autonomous without utilizing the heat amount accumulated inthe insulating material 7 or the like.

The maximum fuel utilization rate Ufmax, on the other hand, is set tochange in a curved line fashion relative to each generating current.Here the range of values which the fuel utilization rate Uf can assumerelative to each generating current (the difference between the maximumfuel utilization rate Ufmax and the minimum fuel utilization rate Ufmin)is narrowest for the maximum generating current, and broadens asgenerating current declines. This is because in the vicinity of maximumgenerating current, the minimum fuel utilization rate Ufmin is high atwhich thermal autonomy is possible, and there is little margin forincreasing the fuel utilization rate Uf (decreasing the fuel supplyamount) even if stored heat is used. Moreover, because the minimum fuelutilization rate Ufmin at which thermal autonomy is possible declines asgenerating current declines, the margin for reducing the fuel supplyamount by utilizing stored heat increases, and when there is a largeamount of stored heat, the fuel utilization rate Uf can be greatlyincreased. Therefore the fuel utilization rate is changed over a largerrange in the region where generated power is small than in the regionwhere generated power is large.

In the region below a predetermined suppressed utilization rategeneration amount IU at which generating current is extremely small, therange of values the fuel utilization rate Uf can assume is set to besmaller as generated power decreases. This means that in the regionwhere generating current is small, the minimum fuel utilization rateUfmin is low at which thermal autonomy is possible, and there is marginfor improvement thereof. However in the region where generating currentis small, the temperature inside the fuel cell module 2 is low,therefore when the fuel utilization rate Uf is greatly improved in thisstate and the stored heat amount accumulated in the insulating material7 or the like is suddenly consumed, there is a risk of inducingexcessive temperature drops inside the fuel cell module 2. Therefore inthe region below, a predetermined suppressed utilization rate generationamount IU at which generating current is extremely small, the changeamount for increasing the fuel utilization rate Uf is greatly suppressedas generated power declines. In other words, the amount of changecausing a reduction in the fuel supply amount as the amount ofgeneration by the fuel cell module 2 declines. The risk of suddentemperature drops can thus be avoided, and the accumulated heat amountcan be utilized over a long time period.

In the present embodiment, the fuel supply amount is reduced by the fueltable change circuit 110 a built into the control section 110 so thatthe fuel utilization rate Uf increases relative to the minimum fuelutilization rate Ufmin. This fuel table change circuit 110 a does notchange the basic fuel supply table, but acts to raise the fuelutilization rate by changing the fuel supply amount which serves asbase, thereby raising the fuel utilization rate.

In step S34 of FIG. 21, the specific values of minimum fuel utilizationrate Ufmin and maximum fuel utilization rate Ufmax are determined usingthe graph in FIG. 24 based on generating current. Next, applying thedetermined minimum fuel utilization rate Ufmin and maximum fuelutilization rate Ufmax to the FIG. 23 graph, the fuel utilization rateUf is determined based on the first integral value N1 id calculated instep S33.

Next, in step S35 of FIG. 21, an air utilization rate is determinedusing the FIGS. 25 and 26 graphs, based on a second integral value N2id.

FIG. 25 is a graph showing setting values for air utilization rate Uarelative to the calculated second integral value N2 id. As shown in FIG.25, when the second integral value N2 id is 0 to 1, the air utilizationrate Ua is set to maximum air utilization rate Uamax, which is themaximum value. In addition, as the second integral value N2 id exceeds 1and increases, the air utilization rate Ua declines, and at secondintegral value N2 id=4 becomes the minimum air utilization rate Uamin,which is a minimum value. Thus the increased portion of air caused byreducing the air utilization rate Ua acts as a cooling fluid, thereforethe setting of the air utilization rate Ua shown in FIG. 25 acts as aforced cooling circuit. The specific values of these minimum airutilization rate Uamin and maximum air utilization rate Uamax aredetermined using the graph shown in FIG. 26, based on generatingcurrent.

FIG. 26 is a graph showing the range of values obtainable for airutilization rate Ua relative to each generating current; the maximum andminimum values for the air utilization rate Ua are shown for eachgenerating current. As shown in FIG. 26, the maximum air utilizationrate Uamax for each generating current is set to increase by a verysmall amount as generating current increases. On the other hand, theminimum air utilization rate Uamin decreases as generating currentincreases. Reducing the air utilization rate Ua (increasing the fuelsupply amount) more than the maximum air utilization rate Uamax resultsin the introduction of a larger amount of air into the fuel cell module2 than is required for generation, causing the temperature inside thefuel cell module 2 to decline. Therefore the air utilization rate Ua isreduced when the temperature in the fuel cell module 2 rises excessivelyand it is necessary to reduce that temperature. In the presentembodiment, reducing the minimum air utilization rate Uamin (increasingthe air supply amount) with the rise in generating current causes theair supply amount corresponding to the minimum air utilization rateUamin to exceed the maximum air supply amount for the generating airflow regulator unit 45 at a predetermined generating current. Thereforein the region in which the minimum air utilization rate Uamin is at orabove the predetermined generating current shown by the dotted line inFIG. 26, there are cases when it is not possible to achieve the airutilization rate Ua set by the graph in FIG. 25. In such cases the airsupply amount actually supplied is set to the maximum air supply amountfor the generating air flow regulator unit 45, regardless of the set airutilization rate Ua. In conjunction with this, the air utilization rateUa which is actually implemented increases at or above a predeterminedgenerating current. When a generating air flow regulator unit with alarger maximum air supply amount is used, the minimum air utilizationrate Uamin for the portion shown by the broken line in FIG. 26 can alsobe achieved. Note that the air utilization rate Ua defined by reachingthe maximum air supply amount for the generating air flow regulator unit45 is described as the limit minimum air utilization rate ULamin.

In step S35 of FIG. 21, the specific values of the minimum airutilization rate Uamin and maximum air utilization rate Uamax aredetermined using the graph in FIG. 26, based on generating current.Next, applying the determined minimum air utilization rate Uamin andmaximum air utilization rate Uamax to the FIG. 25 graph, the airutilization rate Ua is determined based on the second integral value N2id calculated in step S33.

Next, in step S36 of FIG. 21, the ratio S/C of steam amount to carbonamount is determined using FIG. 27, based on the air utilization rate Uadetermined in step S35.

FIG. 27 is a graph in which the horizontal axis shows the airutilization rate Ua and the vertical axis shows the ratio S/C of thesupplied steam amount to the carbon amount contained in the fuel.

First, in the generating current region in which the air utilizationrate Ua set in step S35 is not defined by the maximum air supply amountof the generating air flow regulator unit 45 (between Uamax and ULaminin FIG. 27), the value of the ratio S/C of the steam amount to thecarbon amount is fixed at 2.5. Note that a steam amount to carbon amountratio S/C=1 means that the entire amount of carbon contained in thesupplied fuel is chemically steam reformed by the supplied water(steam), without excess or shortage. Therefore the statement that thesteam amount to carbon amount ratio S/C=2.5 refers to the state in whichsteam (water) is supplied in an amount 2.5 times the minimum steamamount chemically needed to steam reform the fuel. In actuality, at thesteam amount at which S/C=1, carbon deposition occurs inside thereformer 20, so a steam amount at which S/C=approximately 2.5 is theappropriate amount for steam reforming the fuel.

Next, in the generated current region in which the air utilization rateUa set in step S35 is limited by the maximum air supply amount of thegenerating air flow regulator unit 45, the ratio S/C of the steam amountto the carbon amount is determined using the graph in FIG. 27. In FIG.27 the horizontal axis is the air utilization rate Ua; the air supplyamount declines as the air utilization rate Ua increases and approachesthe maximum air utilization rate Uamax. When the air utilization rate Uais reduced, on the other hand, and approaches the minimum airutilization rate Uamin (the dotted line in FIG. 26), the air supplyamount reaches a limit, and the air utilization rate Ua goes to thelimit minimum air utilization rate ULamin. As shown in FIG. 27, when theair utilization rate Ua is larger (air supply amount is low) than thelimit minimum air utilization rate ULamin, the ratio of steam amount tocarbon amount is set at S/C=2.5. Additionally, when the air utilizationrate Ua determined in step S35 is smaller (air supply amount is large)than the limit minimum air utilization rate ULamin (between Uamin andULamin in FIG. 27), the ratio S/C of steam amount to carbon amountincreases as the air utilization rate Ua decreases, and at the minimumair utilization rate Uamin it is set to S/C=3.5. That is, when the airutilization rate Ua determined in step S35 cannot be achieved using thelimit minimum air utilization rate ULamin (when the air utilization rateUa is determined to be within the range of the sloped line in FIG. 26),the ratio of the steam amount to carbon amount S/C is increased, and thewater supply amount is increased. Thus the temperature of the reformedfuel gas discharged from the reformer 20 is reduced, placing thetemperature inside the fuel cell module 2 in a declining trend. Thuswhen the water supply amount is increased after reducing the airutilization rate Ua and increasing the air supply amount, the increasedportion of water (steam) acts as a cooling fluid, so that setting thewater supply amount shown in FIG. 27 acts as a forced cooling circuit.

In step S37 the specific fuel supply amount, air supply amount, andwater supply amount are determined based on the fuel utilization rateUf, air utilization rate Ua, and ratio S/C of steam amount to carbonamount respectively determined in steps S34, S35, and S36. In otherwords, the actual fuel supply amount is calculated by dividing the fuelsupply amount—assuming the entire amount is used for electricalgeneration—by the determined fuel utilization rate Uf, and the actualair supply amount is calculated by dividing the air supplyamount—assuming the entire amount is used for electrical generation—bythe determined air utilization rate Ua. The water supply amount iscalculated based on the calculated fuel supply amount and on the ratioS/C of the steam amount and carbon amount determined in step S36.

Next, in step S38, the control section 110 sends signals to the fuelflow regulator unit 38, the generating air flow regulator unit 45, andthe water flow regulator unit 28 serving as water supply device, andsupplies the amounts of fuel, air, and water calculated in step S37,thereby completing the processing of one iteration of the FIG. 21 flowchart.

Next, we discuss the time intervals at which the FIG. 21 flow chart isexecuted. In the present embodiment the FIG. 21 flow chart is executedevery 0.5 seconds when the output current is large, and as outputcurrent falls, is executed at twice that amount or 1 second, 4 timesthat amount or every 2 seconds, and 8 times that amount or every 3seconds. Thus when the first and second add/subtract values are a fixedvalue, the change in the first and second add/subtract values per unittime becomes more gradual as output current declines. That is, thestored heat estimating circuit 110 b changes the estimated value of thestored heat amount per unit time more suddenly as output current (outputpower) increases. The estimate of the stored heat amount resulting fromthe integral value thus accurately reflects the actual stored heatamount.

Next, referring to FIG. 28, we discuss the procedure for determining thefuel supply amount, air supply amount, and water supply amount when thefuel cell module 2 has degraded. FIG. 28 is a diagram showing generatedvoltage relative to generated current by the fuel cell module 2. Ingeneral there is internal resistance present in the individual fuel cellstack 14, therefore as shown in FIG. 28, the voltage drops when thecurrent output from the fuel cell module 2 increases. The dot-and-dashline shown in FIG. 28 shows the relationship between generated currentand generated voltage when no degradation of the fuel cell module 2 hasoccurred. In contrast, when the fuel cell module 2 does degrade,internal resistance in the individual fuel cell stack 14 rises, sogenerated voltage declines relative to the same generated current.

In the solid oxide fuel cell system of the present embodiment, when thegenerated current drops by 10% or more relative to initial generatedvoltage, and the generated voltage enters the region below the solidline in FIG. 28, the fuel supply amount, air supply amount, and watersupply amount are determined by processing in a manner suited todegradation.

That is, when the generated voltage is in the region below the solidline in FIG. 28, the integration of first integral value N1 id isstopped at step S33 in FIG. 21, and only the integration of secondintegral value N2 id is continued. The value of first integral value N1id used when referring to the FIG. 23 graph to determine the fuelutilization rate Uf is thus fixed at a constant value. The fuelutilization rate Uf is therefore fixed until the generated voltage getsout of the region below the solid line in FIG. 28. Thus changes toincrease the fuel utilization rate Uf are reduced more after degradationof the fuel cell module 2 than prior to degradation of the fuel cellmodule 2. At the same time, the second integral value N2 id used whenreferring to the FIG. 26 graph to determine the air utilization rate Uais reduced as in the past, and increases and decreases in the airutilization rate Ua are continued. The fuel utilization rate Uf ischanged based on degradation of the fuel cell module 2, in addition tothe first and second addition values and power demand corresponding toestimated stored heat amount.

Next, we discuss the operation of a solid oxide fuel cell systemimplemented using the FIG. 21 flow chart.

First, when the value of the first integral value N1 id calculated instep S33 is 0, the fuel utilization rate Uf determined in step S34 isset at the minimum fuel utilization rate Ufmin (fuel supply amountmaximum) for that generated current. Thus even in a state in which thefirst integral value N1 id is 0, and the stored heat accumulated in theinsulating material 7 or the like is small, sufficient fuel is suppliedfor the fuel cell module 2 to achieve thermal autonomy. When the valueof the second integral value N2 id calculated in step S33 is 0, as forthe first integral value N1 id, the air utilization rate Ua determinedin step S35 is set at the maximum air utilization rate (air supplyamount minimum) for that generated current. Therefore cooling of theindividual fuel cell stack 14 can be minimized by the generating airintroduced into the fuel cell module 2, and the temperature of theindividual fuel cell stack 14 can be placed on a rising trend.

Next, when the fuel cell module 2 is operated in a state whereby thedetected temperature Td is higher than the appropriate temperatureTs(I), and Td>Ts(I)+Te, the value of the first add/subtract value M1becomes positive, and the value of first integral value N1 id becomesgreater than 0. Thus in FIG. 23, a fuel utilization rate Uf higher thanthe minimum fuel utilization rate Ufmin is set and the fuel supplyamount is reduced, and the amount of residual fuel remaining and notused to generate electricity is reduced. The fuel utilization rate Uf isgreatly increased by the control section 110 as the value of the firstintegral value N1 id corresponding to the estimated stored heat amountincreases. By increasing the fuel utilization rate Uf, the fuel supplyamount is reduced to below a supply amount at which thermal autonomy ispossible, and high efficiency control utilizing the heat amountaccumulated in the insulating material 7 and the like is executed. Theamount of residual fuel is reduced and the heat amount accumulated inthe insulating material 7 or the like is utilized, therefore the fueltable change circuit 110 a suppresses the rise of the temperature in thefuel cell module 2 while continuing to generate electricity. Whenoperation is continued in the Td>Ts(I)+Te state, addition of thepositive value first add/subtract value M1 is repeated, and the value ofthe first integral value N1 id also increases. When the first integralvalue N1 id reaches 1, the fuel utilization rate Uf (fuel supply amountminimum) is set to the maximum fuel utilization rate Uafmax. Fuelsupplied to the fuel cell module 2 is determined based on the pasthistory of the detected temperature Td, which reflects the heat amountaccumulated in the insulating material 7 or the like.

Even when the first integral value N1 id further increases and exceeds1, the fuel utilization rate Uf is maintained at the maximum fuelutilization rate Uafmax (fuel supply amount minimum), as shown in FIG.23. On the other hand, when the second integral value N2 id, which takesthe same value as the first integral value N1 id (when the fuel cellmodule 2 is not degraded), also exceeds 1, so the air utilization rateUa declines (air supply amount increases) based on FIG. 25. Thus theinside of the fuel cell module 2 is placed in a cooling trend due to theincrease of supplied air.

By contrast, when the fuel cell module 2 is operated in a state wherebythe detected temperature Td is lower than the appropriate temperatureTs(I), and Td<Ts(I)−Te, the value of the first add/subtract value M1becomes negative, and the value of first integral value N1 id isreduced. The fuel utilization rate Uf is therefore maintained (firstintegral value N1 id>1) or decreased (first integral value N1 id≦1).Also, the air utilization rate Ua increases (second integral value N2id>1) or is maintained (second integral value N2 id≦1). The temperatureinside the fuel cell module 2 can thus be set on a rising trend.

The above describes the operation of a solid oxide fuel cell systemfocusing only on the first add/subtract value M1 calculated based on thehistory of the detected temperature Td, but first integral value N1 idand second integral value N2 id are also influenced by the secondadd/subtract value M2. The heat capacity of the fuel cell module 2, andin particular the individual fuel cell stack 14, is extremely large, andchanges in the detected temperature Td thereof are extremely sluggish.Therefore once the detected temperature Td enters a rising trend, it isdifficult to suppress that temperature rise in a short time period, andwhen the detected temperature Td enters a falling trend, as well, a longperiod of time is required to return it to a rising trend. Therefore arising or falling trend appearing in the detected temperature Tdrequires an immediate response by modifying the first and secondintegral values.

That is, when the latest detected temperature Td is above the detectedtemperature Tdb one minute prior by an amount equal to or greater thanthe second add/subtract value threshold value temperature, the secondadd/subtract value M2 becomes a positive value, and the first and secondintegral values are increased. This allows the fact that the detectedtemperature Td is in a rising trend to be reflected in the first andsecond integral values. Similarly, when the latest detected temperatureTd is above the detected temperature Tdb one minute prior by an amountequal or greater than the second add/subtract value threshold valuetemperature, the second add/subtract value M2 becomes a negative value,and the first and second integral values are decreased. In other words,the second add/subtract value M2, which is the quick response estimatevalue, is calculated by the change temperature difference, which is thedifference between the latest detected temperature Td detected by thegenerating chamber temperature sensor 142 and the past detectedtemperature Tdb. Therefore when the detected temperature Td is suddenlydropping, the change amount increasing the fuel utilization rate Uf ismore suppressed than when it is gradually dropping, and since in theregion where generated power is below the suppressed utilization rategeneration amount IU, the maximum fuel utilization rate Ufmax is alsoset low, and the change amount is greatly suppressed. This allows thefact that the detected temperature Td is in a falling trend to bereflected in the first and second integral values. Thus in the presentembodiment, the stored heat amount is estimated based on an integralvalue for the add/subtract determined based on detected temperature, andon the differential value between the newly detected temperature andpast detected temperatures. That is, in the present embodiment, thestored heat amount is estimated by the stored heat estimating circuit110 b based on the integral value of the first add/subtract value M1,which is a basic estimated value calculated based on the history ofdetected temperatures Td, and based on the second add/subtract value M2,which is a quick response estimate value calculated based on the rate ofchange in the detected temperature Td over a period of time shorter thanhistory of the basic estimated value calculation. Thus in the presentembodiment, the stored heat amount is estimated based on the sum of thebasic estimated value and the quick response estimate value.

Note that temperature changes in the fuel cell module 2 are extremelyslow compared to the 1 minute detection interval for detectedtemperatures Td and Tdb, so it is often the case that the secondadd/subtract value M2 is 0. Therefore the first and second integralvalues are primarily dominated by the first add/subtract value M1, andthe second add/subtract value M2 acts to modify the values of the firstand second integral values when a rising or falling trend appears in thedetected temperature Td. Thus in addition to the detected temperaturehistory, changes in recent detected temperatures Td are also added tothe stored heat amount estimated value using the second add/subtractvalue M2. Therefore when the change in recent detected temperatures Tdis large (a change equal to or larger than the second add/subtract valuetemperature recovery temperature) the second add/subtract value M2 has avalue, but the stored heat amount estimated value is modified, and thefuel utilization rate Uf is greatly changed.

Next, referring to FIG. 29 through 32, we discuss limitations to thevariable range of generated power.

As described above, in the solid oxide fuel cell system of the presentembodiment, utilizing the heat amount accumulated in the insulatingmaterial 7 or the like allows the fuel utilization rate to be increasedand the temperature inside the fuel cell module 2 to be controlled to anappropriate temperature by actively utilizing stored heat. As explainedusing FIGS. 19 and 20, frequent increases and decreases in the powerproduced by the fuel cell module 2 to match the power demand can causethe temperature inside the fuel cell module 2 to rise excessively. It ispossible to suppress such excessive temperature rises by increasing thefuel utilization rate and actively utilizing the heat amount accumulatedin the insulating material 7 or the like. As explained using FIG. 24,however, in the region where the power generated is large, the setminimum fuel utilization rate Ufmin is a large value, so there is littleroom for increasing the fuel utilization rate and utilizing stored heat.Therefore when generated power is large, it is difficult to effectivelyreduce an excessively raised temperature inside the fuel cell module 2even by increasing the fuel utilization rate and utilizing stored heat.For this reason, when an excessive temperature rise occurs in the fuelcell module 2 in the present embodiment, the variable range in whichgenerated power is made to follow power demand is restricted to a lowlevel. Since this causes the fuel cell module 2 to be operated at asmall generated power, the margin for utilizing stored heat increases,making it possible to effectively lower the temperature inside the fuelcell module 2. By narrowing the variable range in which generated poweris made to follow power demand, temperature rises caused by frequentincreases and decreases in generated power are suppressed.

Note that temperature rises inside the fuel cell module 2 caused byfrequent increases and decreases in power demand, as explained in FIGS.19 and 20, also occur in the solid oxide fuel cell system of the firstembodiment of the present invention described above. Therefore,referring to FIG. 29 through 32, the limitation on the variable range ofgenerated power explained below can be implemented in combination withthe above-described first embodiment of the present invention.

FIG. 29 is a flowchart showing a procedure for limiting the range ofpower produced by the fuel cell module in the present embodiment. FIG.30 is a map showing current limits versus generated current and detectedtemperature Td. FIG. 31 is a timing chart showing an example of theeffect of the second embodiment of the present invention. FIG. 32 is agraph showing an example of the relationship between temperature insidethe fuel cell module and maximum generatable power.

First, as shown by the solid line in FIG. 30, in the solid oxide fuelcell system of the present embodiment an appropriate temperature is setin the fuel cell module 2 for each generated current. This appropriatetemperature corresponds to the dot-and-dash line in FIG. 22. As shown inFIG. 30, a current maintaining region is set in the region where thetemperature is above the appropriate temperature. The minimumtemperature in this current maintaining region is set to differaccording to generated power from the fuel cell module 2, and thecurrent maintaining region minimum temperature is set to increase asgenerated power increases. The minimum temperature in the currentmaintaining region with respect to each generated power is set so thatthe difference relative to the appropriate temperature for the fuel cellmodule 2 increases as generated power decreases. When the operatingstate of fuel cell module 2 enters this current maintaining region, theoutput current from the fuel cell module 2 is prohibited fromincreasing. Furthermore, a current reduction region is set in the regionwhere the temperature is higher than the current maintaining region.When the operating state enters this current reduction region, theoutput current from the fuel cell module 2 is forcibly reduced. An aircooling region is set in the region where the temperature is above thecurrent reduction region. When the operating state enters this aircooling region, the generating air supply amount is set to the maximumflow volume suppliable by the generating air flow regulator unit 45. Astop operation region is set in the region in which the temperature ishigher than the air cooling region. When the operating state enters thisstop operation region, power generation by the fuel cell module 2 isstopped to prevent malfunctioning of the solid oxide fuel cell system.

Moreover, when the detected temperature Td has risen suddenly, thetemperature demarcating the current maintaining region is lowered asshown by the dot-and-dash line in FIG. 30. In such cases, thetemperature demarcating a current reduction region is lowered as shownby the double dot-and-dash line in FIG. 30. Thus when the detectedtemperature Td suddenly rises, current limiting is quickly implemented,thereby reliably suppressing excessive temperature rises.

Next, referring to FIG. 29, we discuss the procedure for limitingcurrent produced by the fuel cell module.

First, the detected temperature Td is read in step S411 of FIG. 29.Next, in step S42, the detected temperature Td read in step S41 and thedetected temperature Td from a predetermined previous time are compared.If the difference between the detected temperature Td read in step s41and the detected temperature Td from a predetermined previous time is ator below a predetermined threshold temperature, the system advances tostep S43.

In step S43, the basic characteristics shown by the solid line in FIG.30 are selected as a map for determining a temperature region. At thesame time, if the difference between the latest detected temperature Tdand the detected temperature Td at a predetermined previous time islarger than the predetermined threshold temperature, the system advancesto S44; in step S44 the suddenly rising temperature characteristicsshown by the dot-and-dash line and the double dot-and-dash line in FIG.30 is selected as the map for determining the temperature region.

Next, in step S45, a judgment is made as to whether the detectedtemperature Td is within the stop operation region. In this embodiment,if the detected temperature Td is at or above 780° C., the temperatureis judged to be in the stop operation region. When it is judged that thedetected temperature Td is within the stop operation region, the systemadvances to step S46. In step S46, power generation by the fuel cellmodule 2 is stopped, and an emergency stop of the solid oxide fuel cellsystem is effected.

On the other hand, if it is judged in step S45 that the detectedtemperature Td is not within the stop operation region, the systemadvances to step S47. In step S47 a judgment is made as to whether thedetected temperature Td is within the air cooling region. In the presentembodiment when the detected temperature Td is 750° C. or greater, it isjudged to be in the air cooling region. When it is judged that thedetected temperature Td is within the air cooling region, the systemadvances to step S48.

In step S48, the generated current is fixed at the minimum current of 1A; this current is consumed by the auxiliary unit 4 without being outputto the inverter 54. The generating air supply amount is set to themaximum flow volume suppliable by the generating air flow regulator unit45. The water supply amount is also increased and the steam and carbonamounts set to a ratio of S/C=4, completing the processing of oneiteration in the FIG. 29 flow chart.

On the other hand, if it is judged in step S47 that the detectedtemperature Td is not within the air cooling region, the system advancesto step S49. In step S49 a judgment is made of whether the detectedtemperature Td and the generated current are within the currentreduction region; if within the current reduction region, the systemadvances to step S50.

In step S50, the generated current from the fuel cell module 2 is forcedto 4 A or below. In other words, the upper limit value of the generatedpower from the fuel cell module 2 is reduced to a temperaturerise-suppressing power (400 W), which is higher than ½ the maximum ratedpower of 700 W. Thereafter when power demand declines, the upper limitvalue of the generated power (current), following the power demand, isreduced, and generated current is maintained, not increased, even ifpower demand grows. One iteration of the processing in the flow chart ofFIG. 29 is thus completed. This type of limitation on generated currentcontinues until the detected temperature Td and generated current gooutside the current reduction region.

On the other hand, if a judgment is made in step S49 that the detectedtemperature Td and the generated current are not within the currentreduction region, the system advances to step S51. In step S51 ajudgment is made of whether the detected temperature Td and thegenerated current are within the current maintain region; if within thecurrent reduction region, the system advances to step S52.

In step S52, increases in the generated current are prohibited, andthereafter the generated current is maintained without increase, even ifpower demand grows. Subsequently when power demand declines, the upperlimit value of generated current (power), following the drop in powerdemand, is reduced, and the upper limit of generated current (power) ismaintained, not raised, even if power demand grows. This type oflimitation on generated current continues until the detected temperatureTd and generated current go outside the maintain current region and theexcessive temperature rise of the fuel cell module 2 is resolved. Oneiteration of the processing in the flow chart of FIG. 29 is thuscompleted.

In the present embodiment, restrictions on generated power are startedwhen the detected temperature Td exceeds the maintain current regionminimum temperature for each of the generated currents, so the minimumtemperature in the maintain current region relative to each generatedcurrent is referred to as the generated power restriction temperature(FIG. 30). This generated power restriction temperature is set in thepresent embodiment to be higher than the fuel utilization rate changetemperature (Ts(I)+Te) (FIG. 2) at which a change to increase the fuelutilization rate is started.

On the other hand, if a judgment is made in step S51 that the detectedtemperature Td and the generated current are not within the maintaincurrent region, the system advances to step S53. At step S53, limits ongenerated current are not executed, and control utilizing stored heat isexecuted.

Next, referring to FIG. 31, we discuss an example of a generated currentlimitation.

The timing chart shown in FIG. 31 schematically depicts, in order fromthe top, changes in detected temperature Td, target current, generatedcurrent, fuel supply amount, fuel utilization rate, and air supplyamount. Target current here refers to the current obtained from powerdemand and generated voltage.

First, at time t20 in FIG. 31, the generated current is approximately 6A, and the detected temperature Td is in a state slightly below theappropriate temperature at a generated current of 6 A (corresponding tot20 in FIG. 30).

Next, at times t20-t21, because of the repeated larger increases anddecreases in power demand over a short period, target current alsogreatly increases and decreases, and generated current also increasesand decreases to follow. By contrast, the fuel supply amount, asexplained in FIG. 20, is held for a predetermined time after thegenerated current has declined, and is increased ahead of the increasein generated current, thus becoming excessive relative to generatedcurrent, resulting in a large amount of surplus fuel. This surplus fuelis used to heat the interior of the fuel cell module 2, therefore attimes t20-t21 the detected temperature Td is in a rising trend.

Furthermore, at time t21 the detected temperature Td reaches themaintain current region temperature for a generated current ofapproximately 6 A (t21 in FIG. 30, corresponds the transition from stepS51→S52 in FIG. 29). Step S52 in FIG. 29 is thus executed; thereafterincreases in generated current are prohibited, and generated current ismaintained. Therefore at times t21-t22 the target current is growing toapproximately 7 A, but the generated current is maintained atapproximately 6 A. By prohibiting increases in generated current, theupper limit value of the variable range of generated power is reduced,and the variable range is narrowed, causing the amount of residual fuelassociated with changes in power demand to decline. Thus step S52 ofFIG. 29, where the residual fuel amount is reduced while continuingelectrical generation, is acting as a temperature rise suppressingcircuit. Step S51 judges whether step S52, which acts as a temperaturerise-suppressing circuit, is executed, and acts as an excess temperaturerise estimating circuit for estimating the occurrence of excessivetemperature rises in the fuel cell module 2.

At times t21-t22, moreover, the detected temperature Td rises, thereforethe first add/subtract value M1 becomes a truly large value, and thefirst integral value N1 id value also increases remarkably. The fuelsupply amount is thus reduced to increase the fuel utilization rate Uf(FIG. 23). This increase in the fuel utilization rate Uf also acts toreduce the amount of residual fuel and lower the temperature inside thefuel cell module, thus acting as a temperature rise-suppression circuit.Note that at times t21-t22, the fuel utilization rate Uf is increasedand the heat amount accumulated in the insulating material 7 or the likeis actively consumed, but because the thermal capacity of the fuel cellmodule 2 is extremely large, the detected temperature Td continues torise.

Next, at time t22, the increased fuel utilization rate Uf reaches themaximum fuel utilization rate Ufmax (=75%), which is the maximum fuelutilization rate at a generated current of approximately 6 A (firstintegral value N1 id=1 in FIG. 23; FIG. 24). At time t22 the fuelutilization rate Uf is raised up to the maximum fuel utilization rateUfmax, therefore at times t22-t23, the fuel utilization rate Uf ismaintained at the maximum fuel utilization rate Ufmax. On the other handat times t22-t23, the detected temperature Td is still continuing torise, therefore the value of the second integral value N2 id (same valueas the first integral value N1 id) also grows. Associated with this, theair utilization rate Ua is reduced (N2 id in FIG. 25>1); i.e., the airsupply amount is increased.

In addition, at time t23 the detected temperature Td reaches the currentreduction region at a generated current of approximately 6 A(corresponding to step S49→S50 in FIG. 29). This causes step S50 in FIG.29 to be executed, with the generated current suddenly reduced fromapproximately 6 A to 4 A (t23→t23′ in FIG. 30), the upper limit value ofthe generated power variable range further reduced, and the variablerange further narrowed. Therefore the fuel utilization rate Uf isreduced very slightly from the maximum fuel utilization rate Ufmax at agenerated current of 6 A to a maximum fuel utilization rate Ufmax at agenerated current of 4 A (FIGS. 24, 31). Note that at time t23, the fuelutilization rate Uf is lowered, but since the generated current isreduced to 4 A, the absolute amount of the fuel supply amount and theabsolute amount of the residual fuel are lowered. Since the fuelutilization rate Uf is maintained at a maximum fuel utilization rateUfmax with the generated current in a reduced state, consumption of theaccumulated heat amount is further promoted. By reducing generatedcurrent in this way, step S50 in FIG. 29, which reduces the amount ofresidual fuel while continuing generation, also acts as a temperaturerise-suppression circuit. However detected temperature Td still rises attimes t23-t24.

Next, at time t24 the detected temperature Td reaches the air coolingregion temperature (step S47→S48 in FIG. 29, corresponding to t24 inFIG. 30). Step S48 in FIG. 29 is thus executed, and the air supplyamount is increased to the maximum air supply amount for the generatingair flow regulator unit 45. Generated current is gradually reduced from4 A to 1 A. Thereafter, the generated current reduced to 1 A, which isthe temperature rise-suppressing generation amount, is maintained at afixed level until the detected temperature Td declines to a temperaturebelow the current maintain region. The generated current which has beenreduced to 1 A is entirely consumed by the auxiliary unit 4 and notoutput to the inverter 54. With the drop in generated current, the fuelutilization rate Uf is reduced from the maximum fuel utilization rateUfmax at a generated current of 4 A to a maximum fuel utilization rateUfmax (=50%) at a generated current of 1 A (FIG. 24).

Thus at step S50 in FIG. 29, which is the temperature rise-suppressioncircuit, temperature rises reducing the amount of residual fuel aresuppressed, following which, when suppression of further temperaturerises is required, supplied air is increased. The portion of airincreased beyond the supply amount needed for electrical generation actsas a cooling fluid flowing into the fuel cell module 2, so step S48 inFIG. 29 functions as a forced cooling circuit.

On the other hand if by executing step S50, which is the temperaturerise-suppression circuit for reducing the amount of residual fuel, thedetected temperature Td drops without reaching the temperature of theair cooling region, cooling by step S48, which is the forced coolingcircuit, is not executed. Therefore a determination of whether or not toexecute a suppression of temperature rises by the forced cooling circuitis made based on temperature changes in the fuel cell module 2 aftersuppressing rises by the temperature rise-suppression circuit.

After time t24 the rise in detected temperature Td is continued, but attime t25 this shifts to a decline (t24→t25 in FIG. 30). Thereafter thedetected temperature Td declines, and at time t26 declines to the upperlimit temperature of the current reduction region (t25→t26 in FIG. 30).Reduction of the air supply amount is thus begun.

Next, at time t27, the temperature declines to the upper limittemperature of the maintain current region (t26→t27 in FIG. 30). Thedetected temperature Td continues to further decline, and at time t28declines to the lower limit temperature of the maintain current region(t27→t28 in FIG. 30).

At time t28, when the temperature drops leaves the current maintainregion, the generated current begins to increase in order to follow thetarget current. In conjunction with this, the fuel supply amount alsoincreases. The fuel utilization rate Uf increases, adopting a maximumfuel utilization rate Ufmax corresponding to each generated current.

Note that in the above-described embodiment, temperature rises weresuppressed by lowering the upper limit of the generated power variablerange according to the temperature inside the fuel cell module 2, but itis also possible as a variant example to suppress temperature rises bylowering the frequency of increases and decreases in generated power.That is, when the temperature inside the fuel cell module 2 has risen,further temperature rises can be suppressed by reducing following of therise in power demand and reducing following characteristics whichincrease generated power. When following characteristics relative toincreases in power demand are reduced, generated power increases moresluggishly when power demand increases. Therefore when the power demandhas increased and decreased often, the range of increase and decrease ingenerated power attempting to follow this becomes smaller, and thefrequency of increases and decreases is also reduced, so that the amountof residual fuel occurring similarly declines. Therefore the decline infollowing characteristics relative to the increase in power demandcontinues until the excessive temperature rise inside the fuel cellmodule 2 is eliminated.

Alternatively, limits can also be placed on the frequency per unit timeat which the generated power is increased to follow increases in thepower demand. In this case a limitation is placed on the number of timesper predetermined time unit which generated power can switch to a risingtrend; when the number of times per predetermined time is large,generated power is controlled so as not to allow generated power tofollow the increase in power demand.

In the above-described embodiment, the generated current upper limit waslowered to 4 A when the detected temperature Td reached the currentreduction region, but it is also possible, as a variant example, to makevariable the upper limit of the generated power. For example, thegenerated power upper limit value, which is reduced more as thetemperature inside the fuel cell module 2 increases, can be set low.

Next, referring to FIG. 32, we discuss the relationship between thetemperature inside the fuel cell module 2 and maximum generatable power.

As discussed above, there is a correlation between fuel cell module 2generated power (current) and appropriate temperature inside the fuelcell module 2, and to obtain a large generated power requires raisingthe temperature inside the fuel cell module 2. However when the fuelcell module 2 is in a temperature region over 700° C., which is higherthan the appropriate temperature relative to generated power, thecharacteristics of the individual fuel cell stack 14 are such that thepotential produced by the fuel cell units 16 declines. Therefore when alarge current is extracted from the individual fuel cell stack 14 inorder to obtain a large power, the individual fuel cell stack 14temperature further rises, and the potential produced falls, resultingin the phenomenon that output power does not increase even thoughcurrent is increased. As a result, in the region in which thetemperature in the fuel cell module 2 is high, the generatable maximumpower actually declines when the temperature rises, as shown in FIG. 32.When an attempt is made to extract the maximum rated power from the fuelcell module 2 in this type of temperature region, current is increasedin order to increase extracted power; this current rise furtherincreases the temperature of the fuel cell module 2 and reduces thepower extracted from same. When such a state continues, attempting toobtain a predetermined rated power tends to induce a thermal runawaycausing a sudden temperature rise in the fuel cell module 2.

In the present embodiment, thermal runaways can be prevented in advancein the region where the temperature inside the fuel cell module 2 ishigher than the appropriate temperature by maintaining or lowering thegenerated current even when the power demand has increased.

Next, referring to FIG. 33, we discuss measurement of the detectedtemperature Td in the present embodiment.

FIG. 33 is a flow chart showing a procedure for calculating a firstadd/subtract value M1 based on temperatures Td detected by multipletemperature sensors.

As shown in FIG. 3, in the present embodiment two generating chambertemperature sensors 142 are provided inside the generating chamber 10.Here, inside the fuel cell module 2 in the present embodiment, twentyfuel cell units 16 are arrayed in the width direction (FIG. 2) and eightfuel cell units 16 are arrayed in the depth direction (FIG. 3).Therefore a total of 160 fuel cell units 16 are arranged in a rectangleas seen in plan view. In this embodiment, of the two generating chambertemperature sensors 142, one is disposed adjacent to the vertex of therectangle, and the other is disposed adjacent to the midpoint of thelong side of the rectangle. Thus in the present embodiment, the twogenerating chamber temperature sensors 142 are disposed so thatdifferent temperatures are detected within the fuel cell module 2.

Therefore the temperature Td detected by the generating chambertemperature sensors 142 disposed adjacent to the rectangle verticesprimarily reflect the temperature of the fuel cell units 16 disposednear the vertices of the rectangle, and the temperature Td detected bythe generating chamber temperature sensors 142 disposed adjacent to themidpoint of the long side of the rectangle primarily reflect thetemperature of the fuel cell units 16 disposed near the midpoint of thelong side of the rectangle. The fuel cell units 16 disposed close to thevertices of the rectangle can easily be robbed of heat by thesurrounding insulating material 7 or the like and are therefore at thelowest temperature; the fuel cell units 16 disposed near the midpoint ofa long side of the rectangle reach a higher temperature than the fuelcell units 16 disposed near the vertices. In the present embodiment, thetemperature differences between fuel cell units 16 may reach severaltens of degrees. Note that it is believed that the fuel cell units 16disposed near the intersection of the diagonals of the rectangle reachthe highest temperature, and generating chamber temperature sensors mayalso be disposed so as to measure this temperature.

In step S61 of FIG. 33, detected temperatures Td are respectively readin from the two generating chamber temperature sensors 142. Next, instep S62, an average value for the read-in detected temperature Td iscalculated and a judgment made of whether the averaged temperature ishigher than the appropriate temperature Ts(I). When the averagedtemperature is higher than the appropriate temperature Ts(I), the systemadvances to step S63; when it is lower than the appropriate temperatureTs(I), the system advances to step S64.

In step S63, a first add/subtract value M1 is calculated based on thehigher of the two detected temperatures Td (the first add/subtract valueM1 becomes a positive value or 0) and processing for one iteration ofthe FIG. 33 flow chart is completed. That is, the estimated amount ofincrease in the stored heat amount is determined based on the higher ofthe two detected temperatures Td. In step S64, on the other hand, afirst add/subtract value M1 is calculated based on the lower of the twodetected temperatures Td (the first add/subtract value M1 becomes anegative value or 0) and processing for one iteration of the FIG. 33flow chart is completed. That is, the estimated amount of reduction inthe stored heat amount is determined based on the lower of the twodetected temperatures Td. Thus the detected temperature Td on the hightemperature side is adopted when above the appropriate temperatureTs(I), whereas the low side detected temperature Td is adopted whenbelow the appropriate temperature Ts(I). The stored heat amount is inthis way estimated based on the temperature of the higher temperaturefuel cell unit 16 when excessive temperature rise becomes a problem.When temperature reduction is a problem, the stored heat amount isestimated based on the low temperature individual fuel cell unit 16(normally the fuel cell units positioned at vertices of the rectangle),therefore the stored heat amount can be estimated on the safe side evenwhen the temperatures of each of the fuel cell units 16 differ.

Note that in the above-described embodiment the detected temperature Tdon either the high temperature side or the low temperature side wasselected and an integral value calculated based thereon, but it is alsoacceptable as a variant example to obtain the respective integral valuesfor each detected temperature Td. That is, if stored heat amounts areestimated by determining add/subtract values for each of multipledetected temperatures and integrating the determined add/subtract valuesfor each detected temperature to calculate multiple integral values, andif the largest numerical value of the multiple integral values isselected when all of the multiple integral values are increasing, andthe smallest numerical value of the multiple integral values is selectedwhen a portion of the multiple integral values are decreasing, thisvalue may be used as the stored heat amount estimated value.

In the above-described present embodiment, we adopted the hightemperature side of the detected temperature in step S63 and the lowtemperature side of the detected temperature in step S64, but is alsopossible as a variant example to calculate the first add/subtract valueM1 based on a weighted average of the two detected temperatures as a wayof estimating the stored heat amount. For example, in step S63 the firstadd/subtract value M1 can be calculated based on a value obtained byadding a value 0.7 times the detected temperature on the hightemperature side to a value 0.3 times the detected temperature on thelow temperature side, then in step S63 the first add/subtract value M1can be calculated based on a value obtained by adding a value 0.3 timesthe detected temperature on the high temperature side to a value 0.7times the detected temperature on the low temperature side. Thus in stepS63, where the detected temperature Td is high and the estimated valueof the stored heat amount is increased (first add/subtract value M1 ispositive or 0) the highest of the multiple detected temperatures Td isused as the most heavily weighted factor for estimating the stored heatamount, whereas in step S64, where the estimated value of the storedheat amount is decreased (first add/subtract value M1 is negative or 0)the lowest temperature is used as the most heavily weighted factor forestimating the stored heat amount.

It is also possible to at all times calculate the first add/subtractvalue M1 from the simple average of each detected temperature Td,without weighting each of the detected temperatures Td.

When the temperature of fuel cell units positioned at vertices of therectangle has dropped to or below a predetermined usage-suppressing cellunit temperature, the first add/subtract value M1 can be determined in away which suppresses increases in the fuel utilization rate Uf.

Next, referring to FIG. 34, we discuss calculation of an add/subtractvalue according to a variant example of the present embodiment. Notethat calculation of the add/subtract value according to this variantexample may be used together with the processing in FIG. 33, or may beapplied on its own. When applying the present variant example on itsown, it is acceptable to have one generating chamber temperature sensor142.

FIG. 34 is a flow chart showing the procedure for calculating a firstadd/subtract value M1 based on the detected temperature from thereformer temperature sensor 148, which is another temperature detectiondevice in addition to the generating chamber temperature sensors 142serving as temperature detection device.

First, in step S71 of FIG. 34, detected temperatures are read in fromthe reformer temperature sensor 148. In the present embodiment, thereare reformer temperature sensors 148 attached at two locations, on theentrance side and the exit side of the reformer 20, so that temperaturesin the vicinity of the entrance and exit of the reformer 20 aremeasured. Normally the temperature of the reformer 20 is low on theentrance side where the endothermic steam reforming reaction occursabundantly, and high on the exit side.

Next, in step S72 each detected temperature in the reformer 20 iscompared to a predetermined usage-suppressing reformer temperature.First, if the lower detected temperature of the two reformer 20 detectedtemperatures is lower than a low temperature-side usage-suppressingreformer temperature Tr0, and the higher of the detected temperatures islower than high temperature-side usage-suppressing reformer temperatureTr1, the system advances to step S73. On the other hand if the higherdetected temperature of the two reformer 20 detected temperatures ishigher than a high temperature-side usage-suppressing reformertemperature Tr1, and the lower detected temperature is higher than lowtemperature-side usage-suppressing reformer temperature Tr0, the systemadvances to step S75. When neither of these is the case, the systemadvances to step S74.

In step S73, because the temperature of the reformer 20 is lower thaneach of the usage-suppressing reformer temperatures, the firstadd/subtract value M1 is corrected so that the fuel utilization rate Uffalls (the fuel supply amount increases). That is, a value obtained bysubtracting 10% of the absolute value of first addition/subtractionvalue M1 calculated based on the temperature Td detected by thegenerating chamber temperature sensors 142 is used for integration. Thefirst integral value N1 id, which is the estimated value of the storedheat amount, thus decreases (its increase is suppressed), placing thefuel utilization rate Uf on a decreasing trend (the increase in the fuelutilization rate is suppressed), raising the temperature of the reformer20.

In step S75, on the other hand, because the temperature of the reformer20 is higher than each of the usage-suppressing reformer temperatures,the first add/subtract value M1 is corrected so that the fuelutilization rate Uf rises (the fuel supply amount decreases). That is, avalue obtained by adding to the first addition/subtraction value M1 10%of the absolute value of first addition/subtraction value M1 calculatedbased on the temperature Td detected by generating chamber temperaturesensors 142 is used as the first addition/subtraction value M1 forintegration. The first integral value N1 id, which is the estimatedvalue of the stored heat amount, thus increases (its decrease issuppressed), placing the fuel utilization rate Uf in a rising trend, sothat the temperature of the reformer 20 is decreased. Damage to thereformer 20 caused by excessive rise in the reformer 20 temperature isthus prevented.

In step S74, because the reformer 20 is in the appropriate temperaturerange, no correction is made to the first add/subtract value M1, andprocessing of one iteration of the FIG. 34 flow chart is completed.(Since the two detected reformer 20 temperatures are correlated, a statein which the lower detected temperature is lower than lowtemperature-side usage-suppressing reformer temperature and the higherof the detected temperatures is higher than high temperature-sideusage-suppressing reformer temperature normally does not occur.)

Note that in the present embodiment the fuel utilization rate may alsobe corrected by averaging the temperatures detected by the two reformertemperature sensors 148 and comparing the averaged detected temperatureto one or two of the usage-suppressing reformer temperatures. When thechange rate is high in response to the rate of change per unit time inthe temperatures detected by the reformer temperature sensors 148, theamount of correction to the fuel utilization rate may be increased.

Next, referring to FIG. 35, we discuss calculation of an add/subtractvalue according to a variant example of the present embodiment. Notethat calculation of the add/subtract value according to this variantexample may be used together with the processing in FIGS. 33 and 34, ormay be applied on its own. When applying the present variant example onits own, it is acceptable to use one generating chamber temperaturesensor 142.

FIG. 35 is a flow chart showing the procedure for calculating a firstadd/subtract value M1 based on the detected temperature from an exhausttemperature sensor 140, which is another temperature detection device inaddition to the generating chamber temperature sensors 142 serving astemperature detection device.

First, in step S81 of FIG. 35, detected temperatures are read in fromthe reformer temperature sensor 148. In the present embodiment, theexhaust temperature sensor 140 is disposed to measure the temperature ofexhaust gas combusted in the combustion chamber 18 and discharged afterpassing through an exhaust gas discharge pipe 82.

Next, in step S82, the detected temperature of the exhaust gas iscompared to a predetermined usage-suppressing exhaust gas temperature.First, if the detected exhaust gas temperature is below a predeterminedlow temperature side usage-suppressing exhaust temperature Tem0, thesystem advances to step S83. On the other hand, if the detected exhaustgas temperature is above a predetermined high temperature sideusage-suppressing exhaust temperature Tem1, the system advances to stepS85. If the exhaust gas detected temperature is below the hightemperature side usage-suppressing exhaust temperature Tem1 and abovethe low temperature side usage-suppressing exhaust temperature Tem0, thesystem advances to step S84.

In step S83, since the temperature of the exhaust gas is lower than theappropriate temperature, the first add/subtract value M1 is corrected sothat the fuel utilization rate Uf decreases (the fuel supply amountincreases). That is, a value obtained by subtracting from the firstaddition/subtraction value M1 10% of the absolute value of firstaddition/subtraction value M1 calculated based on the temperature Tddetected by generating chamber temperature sensors 142 is used as thefirst addition/subtraction value M1 for integration. The first integralvalue N1 id, which is the estimated value of the stored heat amount,thus decreases (its increase is suppressed), placing the fuelutilization rate Uf on a decreasing trend (the increase in the fuelutilization rate Uf is suppressed), so that the temperature of theexhaust gas is raised.

In step S85, on the other hand, since the temperature of the exhaust gasis higher than the appropriate temperature, the first add/subtract valueM1 is corrected so that the fuel utilization rate Uf increases (the fuelsupply amount decreases). That is, a value obtained by adding to thefirst addition/subtraction value M1 10% of the absolute value of firstaddition/subtraction value M1 calculated based on the temperature Tddetected by generating chamber temperature sensors 142 is used as thefirst addition/subtraction value M1 for integration. The first integralvalue N1 id, which is the estimated value of the stored heat amount,thus increases (its decrease is suppressed), placing the fuelutilization rate Uf in a rising trend, so that the temperature of theexhaust gas is decreased. The temperature inside the fuel cell module 2is by this means made appropriate.

In step S84, because the exhaust gas is in the appropriate temperaturerange, no correction is made to the first add/subtract value M1, andprocessing for one iteration of the FIG. 35 flow chart is completed.

Note that in this variant example, when the change rate is highaccording to the rate of change per unit time in the temperaturesdetected by the exhaust temperature sensor 140, the range of correctionto the fuel utilization rate may be increased.

We have explained above preferred embodiments of the present inventionabove, but various changes may be made to the above-describedembodiments. In particular, in the above-described embodiments thethermal capacity of thermal insulating material was fixed, but a variantexample fuel cell module can also be constituted in which thermalcapacity is variable. In such cases, an additional thermal capacitymember with a large thermal capacity is placed in such a way as to beconnectable and disconnectable from the fuel cell module. In a staterequiring increased thermal capacity, the additional heat capacitymember is thermally connected to the fuel cell module; in a staterequiring decreased thermal capacity, the additional heat capacitymember is thermally disconnected from the fuel cell module. For example,at the time of solid oxide fuel cell system startup, thermal capacity isreduced by disconnecting the additional heat capacity member, and thefuel cell module temperature rise is sped up. On the other hand, when itis anticipated that a solid oxide fuel cell system will operate for longhours at high generated power, the additional heat capacity member isconnected so that the fuel cell module can store a larger surplus heatamount.

The following constitutions of the present invention are also possiblepreferred embodiments.

1. A solid oxide fuel cell system for producing a variable generatedpower in accordance with power demand, having: a fuel cell module forgenerating electricity using supplied fuel; a generating oxidant gassupply device for supplying generating oxidant gas to a fuel cellmodule; a heat storage material for storing heat produced by the fuelcell module; a demand power detection circuit for detecting powerdemand; a controller for determining a fuel supply amount by referringto a basic fuel supply table set so that the fuel utilization rate ishigh when generated power is large and the fuel utilization rate is lowwhen generated power is small, based on demand power detected by thedemand power detection device, and controlling the fuel supply device sothat the determined fuel supply amount is supplied; and a fuel tablechange circuit for changing the basic fuel supply table so that whengenerated power is small, the fuel utilization rate temporarilyincreases during the time when a utilizable heat amount stored duringhigh generated power is stored in the heat storage material, therebyreducing the fuel supply amount.

In the present invention constituted as in 1 above, the fuel supplydevice and generating oxidant gas supply device respectively supply fueland generating oxidant gas to the fuel cell module. The fuel cell modulegenerates electricity using the supplied fuel and generating oxidantgas, and heat produced in the fuel cell module is stored by the heatstorage material. Based on the demand power detected using the demandpower detection device, the controller refers to the basic fuel supplytable, set so that the fuel utilization rate is high when generatedpower is large and the fuel utilization rate is low when generated poweris small, to determine a fuel supply amount and control the fuel supplydevice. When generated power is small, the fuel table change circuitchanges the basic fuel supply table to temporarily increase the fuelutilization rate during the period when the utilizable heat amountstored during high generated power is being stored in the heat storagematerial, thereby reducing the fuel supply amount.

Generally in solid oxide fuel cell system when generated power is small,electrical generation heat declines, making it easier to induce adecline in the temperature of the fuel cell module. Therefore at timesof low power generation, the fuel utilization rate is reduced and fuelnot used for generating electricity is combusted to heat up the fuelcell module and prevent excessive temperature drops. In particular, insolid oxide fuel cell system of the type in which a reformer is disposedwithin the fuel cell module, an endothermic reaction occurs inside thereformer, facilitating an even further reduction in temperature. In thepresent invention thus constituted, the heat amount stored in the heatstorage material during high generated power is actively utilized duringlow generated power, thus enabling a control which raises the fuelutilization rate only during temporary periods during which temperaturedrops can be suppressed, so that overall energy efficiency of the solidoxide fuel cell system can be improved while maintaining thermalself-sufficiency and avoiding excessive temperature drops.

2. In the present invention constituted as described in 1, the fueltable change circuit preferably temporarily executes a change to thebasic fuel supply table, reducing the fuel supply amount when generatedpower is low, then completes the change, and the controller controls thefuel supply device based on the original fuel supply table.

In the present invention constituted as described in 2 above, the fueltable change circuit temporarily executes a change to the basic fuelsupply table, reducing the fuel supply amount when generated power islow, then completes the change; thereafter the fuel supply device iscontrolled based on the basic fuel supply table, thereby reliablyavoiding the risk that the heat amount stored in the heat storagematerial will be excessively reduced so as to cause an extraordinarytemperature drop.

3. In the present invention constituted as described in 2 above, thebasic fuel supply table is preferably set so that at a predeterminedmiddle level generated power, a greater heat amount is stored in theheat storage material, such that heat amounts stored during highgenerated power can be utilized during low generated power.

In the present invention constituted as described in 3 above, the basicfuel supply table is set so that in a region above medium generatedpower, a larger heat amount is stored in the heat storage material,therefore by actively storing heat in a region above the mediumelectrical generation level where the fuel utilization rate can beincreased, this heat can be consumed during low power generation whenthe fuel cell module temperature is relatively low and self-sustainingis difficult, so that high efficiency operation at a high fuelutilization rate with effective use of the stored heat amount can bereliably carried out.

4. In the present invention constituted as described in 3 above, thebasic fuel supply table is preferably set so that in a region in whichthe generated power is higher than the middle value of the generatedpower range, a larger heat amount is stored in the heat storagematerial.

In the present invention constituted as described in 4 above, a largeramount of heat is stored in the heat storage material in the regionwhere generated power is greater than the middle value of the generatedpower range. Therefore in the vicinity of the middle value of thefrequently used generated power range, the amount of stored surplus heatis suppressed, and a large amount of heat is stored in the heat storagematerial during power demand peaks. Thus when a solid oxide fuel cellsystem is used in a residence, excessive fuel consumption to store largeheat amounts is suppressed during the periods of most frequent powerdemand amounts, being the medium level power demand amounts occurringduring the day, etc., while on the other hand large heat amounts arestored during time periods with peak power demand, such as eveninghours, so that heat amounts stored in the evening hours are immediatelyconsumed in the follow-on late night period; hence wasteful storage ofheat amounts over long periods is eliminated, and a high efficiencyoperation can be achieved to reliably take effective advantage of storedheat during the late night period when generated power is greatlyreduced.

5. In the present invention constituted as described in 3 above, thereis preferably furthermore a stored heat estimating circuit forestimating the amount of stored heat in the heat storage material, andthe fuel table change circuit executes changes to the basic fuel supplytable when the stored heat amount estimated by the stored heatestimating circuit is equal to or greater than a predeterminedchange-executed stored heat amount, and does not change the basic fuelsupply table when the estimated stored heat amount is less than thechange-executed stored heat amount.

In the present invention constituted as described in 5 above, the storedheat amount in the heat storage material is estimated by the stored heatestimating circuit, therefore changes to increase the fuel utilizationrate can be stably executed, and a change is executed when the estimatedstored heat amount is equal to or greater than a predeterminedchange-execution stored heat amount, so that overcooling can be morereliably prevented.

6. In the present invention constituted as described in 5 above, thefuel table change circuit preferably increases the amount of change inthe basic fuel supply table, reducing the fuel supply amount more as thestored heat amount estimated by the stored heat estimating circuitincreases.

In the present invention constituted as described in 6 above, the fuelutilization rate is greatly improved as the estimated stored heat amountincreases, therefore overall energy efficiency can be more safely andgreatly improved.

7. In the present invention constituted as described in 6 above, thefuel table change circuit, based on the stored heat amount estimated bythe stored heat estimating circuit, preferably selects and executes atleast one of either a change in the period over which basic fuel supplytable changes are executed, or a change in the amount of change to thebasic fuel supply table.

In the present invention constituted as described in 7 above, a changeis made in either the period over which change is executed to the basicfuel supply table or the amount of change thereof, therefore overallenergy efficiency can be reliably improved.

8. In the present invention constituted as described in 5 above, thestored heat estimating circuit preferably estimates the amount of storedheat in the heat storage material based on the temperature of the fuelcell module.

In the present invention constituted as described in 8 above, the storedheat amount is estimated based on the temperature of the fuel cellmodule, which is strongly correlated with the amount of stored heat,therefore the stored heat amount can be relatively accurately estimatedwithout provision of particular sensors, and fuel cell moduleperformance degradation, excessive temperature drops, and the like canbe reliably avoided.

9. In the present invention constituted as described in 5 above, thestored heat estimating circuit preferably estimates the amount of storedheat in the heat storage material based on the past operating history ofthe fuel cell module.

In the present invention constituted as described in 9 above, the storedheat amount is estimated based on operating history prior to the startof changes to the basic fuel supply table, therefore compared to acontrol based on instantaneous current temperature alone, a moreaccurate control based on residual stored heat amounts is possible, sothat a safer, simpler, and more accurate estimate of the stored heatamount can be made.

10. In the present invention constituted as described in 9 above, thestored heat estimating circuit estimates the stored heat amount in theheat storage material based on the past generated power of the fuel cellmodule, and on the time over which it was operated at that generatedpower.

In the present invention constituted as described in 10 above, thestored heat amount is estimated based on fuel cell module generatedpower and the time thereof, therefore the stored heat amount can beaccurately estimated without provision of any special sensors.

11. In the present invention constituted as described in 5 above, thefuel table change circuit determines a predetermined change executionperiod based on stored heat estimated by the stored heat estimatingcircuit at the commencement of change in the basic fuel supply table,and executes a change within this change execution period.

In the present invention constituted as described in 11 above, changesare executed within a change execution period determined based on astored heat amount estimated by the stored heat estimating circuit,therefore changes in the basic fuel supply table utilizing stored heatcan be effected using a simpler control.

12. In the present invention constituted as described in 3 above, thereis preferably a change period extension circuit for extending the periodin which, during the execution of a change to the basic fuel supplytable by the fuel table change circuit, a reduction in stored heat inthe heat storage material is suppressed, and the period over whichchanges in the basic fuel supply table are executed is extended.

The present invention constituted as described in 12 above is furnishedwith a change period extension circuit for extending the period forexecuting changes to the basic fuel supply table, therefore the storedheat amount can be effectively utilized in accordance with conditions.

13. In the present invention constituted as described in 12 above, thechange period extension circuit preferably reduces the change amount inthe basic fuel supply table as the period over which changes to thebasic fuel supply table are executed lengthens, in conjunction withdecreases in the amount of heat stored in the heat storage material.

In the present invention constituted as described in 13 above, theamount of change relative to the basic fuel supply table is reduced withthe decrease in stored heat amount, therefore the period during whichthe fuel utilization rate is increased can be extended without inducingexcessive temperature drops in the fuel cell module, degradation ofperformance, or the like.

14. In the present invention constituted as described in 12 above, thechange period extension circuit preferably reduces the basic fuel supplytable change amount more as generated power decreases.

In the present invention constituted as described in 14 above, becausethe change amount relative to the basic fuel supply table is reducedmore as generated power decreases, there is a decrease in the changeamount during low power generation, in which the amount of stored heatutilized increases, and the period during which the fuel utilizationrate is increased can be extended while reliably avoiding excessivetemperature drops in the fuel cell module, degradation of performance,or the like.

15. In the present invention constituted as described in 12 above,during the execution of changes to the basic fuel supply table thechange period extension circuit controls the generating oxidant gassupply device to reduce oxidant gas for generation supplied to the fuelcell module.

In the present invention constituted as described in 15 above,generating oxidant gas supplied to the fuel cell module is reducedduring change execution, therefore the carrying off of the heat amountstored in the heat storage material by oxidant gas can be suppressed,and stored heat can be effectively used over a longer time period.

16. The present invention constituted as described in 3 abovefurthermore preferably has an overcooling prevention circuit forpreventing overcooling of the fuel cell module when the amount of storedheat in the heat storage material is small.

The present invention constituted as described in 3 above is furnishedwith an overcooling prevention circuit, therefore overcooling caused byincreasing the fuel utilization rate can be reliably prevented in astate in which the amount of stored heat has declined.

17. In the present invention constituted as described in 16 above,during the period when changes in the fuel supply amount are beingexecuted by the fuel table change circuit, the overcooling preventioncircuit preferably improves the fuel supply amount followingcharacteristics by the fuel supply device more than the followingcharacteristics during normal operation.

In the present invention constituted as described in 17 above, fuelsupply amount following characteristics are improved during the periodwhen changes in the fuel supply amount are being executed, therefore thefuel supply amount can be quickly increased when the fuel utilizationrate drops in conjunction with a decline in the stored heat amount.Overcooling of the fuel cell module caused by delays in response whichcause an increase in the fuel supply amount can thus be prevented.

What is claimed is:
 1. A solid oxide fuel cell system for producingvariable generated power in accordance with power demand, comprising: afuel cell module that generates power using supplied fuel; a fuel supplydevice that supplies fuel to the fuel cell module; a generating oxidantgas supply device that supplies oxidant gas for electrical generation tothe fuel cell module; a heat storage material that stores heat producedwithin the fuel cell module; a demand power detection device thatdetects power demand; and a controller programmed to control the fuelsupply device based on the demand power detected by the demand powerdetection device so that the fuel utilization rate increases whengenerated power is large and decreases when generated power is small,wherein the controller is programmed to change the electrical poweractually output from the fuel cell module with a delay after changingthe fuel supply amount based on changes in demand power; wherein thecontroller comprises a stored heat estimating circuit that estimates theamount of surplus heat based on fuel supplied by the fuel supply deviceand on the power output at a delay relative to fuel supply, and whereinwhen the stored heat estimating circuit estimates that a utilizableamount of heat has accumulated in the storage material, the controllerreduces the fuel supply amount so that the fuel utilization rate for thesame generated power is increased relative to the case when a utilizableamount of heat has not accumulated.
 2. The solid oxide fuel cell systemof claim 1, wherein the controller greatly raises the fuel utilizationrate as the stored heat amount estimated by the stored heat estimatingcircuit increases.
 3. The solid oxide fuel cell system of claim 2,wherein the controller makes much greater changes in the fuelutilization rate relative to changes in the estimated stored heat amountin the region where the amount of stored heat estimated by the storedheat estimating circuit is large than in the region where the estimatedstored heat amount is small.
 4. The solid oxide fuel cell system ofclaim 2, wherein the stored heat estimating circuit estimates a storedheat amount by summing addition and subtraction values reflecting thesurplus heat amount caused by the output of power at a delay relative tofuel supply.
 5. The solid oxide fuel cell system of claim 4, wherein theaddition and subtraction values are determined based on the temperatureinside the fuel cell module, the surplus heat amount calculated usingthe relationship between fuel supply amount and generated power, theamount of increase/decrease in generated power, or the number of timesgenerated power is increased/decreased per hour.
 6. The solid oxide fuelcell system of claim 2, wherein the controller controls the fuel supplydevice so that when a utilizable amount of heat has not accumulated inthe heat storage material, a greater amount of heat is stored in theheat storage material in a region greater than a predetermined mediumgenerated power, so that heat amounts accumulated during large powergeneration can be utilized during small power generation.
 7. The solidoxide fuel cell system of claim 6, wherein the controller controls thefuel supply device so that a larger amount of heat is stored in the heatstorage material in the region where generated power is greater than themiddle value of the generated power range.
 8. The solid oxide fuel cellsystem of claim 6, wherein the controller increases the fuel utilizationrate when the stored heat amount estimated by the stored heat estimatingcircuit is equal to or greater than a predetermined change executionstored heat amount.
 9. The solid oxide fuel cell system of claim 2,wherein the controller determines a predetermined change executionperiod based on the stored heat amount estimated by the stored heatestimating circuit at the start of high efficiency control at anincreased fuel utilization rate, and executes high efficiency controlwithin this change execution period.
 10. The solid oxide fuel cellsystem of claim 2, further comprising a change period extension circuitfor suppressing decreases in the amount of heat stored in the heatstorage material to extend the period of execution of high efficiencycontrol during execution of high efficiency control at an increased fuelutilization rate.
 11. The solid oxide fuel cell system of claim 10,wherein the change period extension circuit decreases the fuelutilization rate in proportion to the lengthening of the period duringwhich the high efficiency control is executed, in conjunction with thedecrease in the amount of stored heat stored in the heat storagematerial.
 12. The solid oxide fuel cell system of claim 10, wherein thechange period extension circuit decreases the fuel utilization rate inproportion to the decrease in generated power.
 13. The solid oxide fuelcell system of claim 10, wherein the change period extension circuitcontrols the generating oxidant gas supply device to reduce oxidant gasfor generation supplied to the fuel cell module while high efficiencycontrol is being executed.
 14. The solid oxide fuel cell system of claim2, further comprising an overcooling prevention circuit that preventsovercooling of the fuel cell module when the stored heat amount in theheat storage material is small.
 15. The solid oxide fuel cell system ofclaim 14, wherein during execution of high efficiency control with anincreased fuel utilization rate, the overcooling prevention circuitimproves the fuel supply amount following characteristics of the fuelsupply device more than during normal operation.
 16. The solid oxidefuel cell system of claim 2, further comprising a combustion portion forheating the fuel cell module by combusting residual fuel, which isremaining fuel supplied by the fuel supply device and not used for powergeneration; wherein the controller further includes: a power extractiondelay circuit which, when generated power is increased, increases thefuel supply amount supplied to the fuel cell module, then increases thepower extracted from the fuel cell module after a delay; an excesstemperature rise estimating circuit that estimates the occurrence ofexcessive temperature rises inside the fuel cell module; a temperaturerise suppression circuit which, when the occurrence of an excessivetemperature rise is estimated by the excess temperature rise estimatingcircuit, suppresses temperature rises in the fuel cell module whilecontinuing power generation by reducing the residual fuel produced bythe delay of output power provided by the power extraction delaycircuit; and a forced cooling circuit for lowering the temperatureinside the fuel cell module by causing a cooling fluid to flow into thefuel cell module when further temperature rise suppression is requiredafter executing temperature rise suppression using the temperature risesuppression circuit.
 17. The solid oxide fuel cell system apparatus ofclaim 16, wherein the temperature rise suppression circuit controlstemperature rise inside the fuel cell module by increasing the fuelutilization rate; and wherein the controller determines whether or notto execute a temperature rise suppression by the forced cooling circuitbased on changes in the temperature inside the fuel cell module after atemperature rise suppression has been executed by the temperature risesuppression circuit.
 18. The solid oxide fuel cell system apparatus ofclaim 17, wherein the temperature rise suppression circuit increases thefuel utilization rate and suppresses temperature rises in the fuel cellmodule by reducing the frequency with which generated power is increasedand decreased when following fluctuations in demand power.
 19. The solidoxide fuel cell system apparatus of claim 17, wherein the forced coolingcircuit increases the flow amount of oxidant gas supplied by thegenerating oxidant gas supply device and utilizes the additional oxidantgas as a fluid body for cooling.
 20. The solid oxide fuel cell system ofclaim 2, further comprising a combustion portion for heating the fuelcell module by combusting residual fuel, which is remaining fuelsupplied by the fuel supply device and not used for power generation;and a temperature detection device for detecting the temperature of thefuel cell module; wherein the stored heat estimating circuit estimatesthe stored heat amount stored in the heat storage material based on thedetected temperature detected by the temperature detection device;wherein the controller includes a power extraction delay circuit thatincreases the generated power output from the fuel cell module at adelay after increasing the fuel supply amount supplied to the fuel cellmodule when increasing generated power; wherein the controller includesa fuel supply amount change circuit that executes high efficiencycontrol to reduce the fuel supply amount so that the fuel utilizationrate rises, thereby causing the heat amount stored in the heat storagematerial to be consumed; and wherein the controller includes atemperature rise suppression circuit that suppresses temperature risesby reducing the upper limit value in a variable range of power generatedby the fuel cell module.